The integrin alpha L beta 2 hybrid domain serves as a link for the propagation of activation signal from its stalk regions to the I-like domain

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Regulation of cell adhesion: a collaborative effort of integrins, their ligands, cytoplasmic actors, and phosphorylation

Integrins are large heterodimeric type 1 membrane proteins expressed in all nucleated mammalian cells. Eighteen α-chains and eight β-chains can combine to form 24 different integrins. They are cell adhesion proteins, which bind to a large variety of cellular and extracellular ligands. Integrins are required for cell migration, hemostasis, translocation of cells out from the blood stream and further movement into tissues, but also for the immune response and tissue morphogenesis. Importantly, integrins are not usually active as such, but need activation to become adhesive. Integrins are activated by outside-in activation through integrin ligand binding, or by inside-out activation through intracellular signaling. An important question is how integrin activity is regulated, and this topic has recently drawn much attention. Changes in integrin affinity for ligand binding are due to allosteric structural alterations, but equally important are avidity changes due to integrin clustering in the plane of the plasma membrane. Recent studies have partially solved how integrin cell surface structures change during activation. The integrin cytoplasmic domains are relatively short, but by interacting with a variety of cytoplasmic proteins in a regulated manner, the integrins acquire a number of properties important not only for cell adhesion and movement, but also for cellular signaling. Recent work has shown that specific integrin phosphorylations play pivotal roles in the regulation of integrin activity. Our purpose in this review is to integrate the present knowledge to enable an understanding of how cell adhesion is dynamically regulated.

On the contributing role of the transmembrane domain for subunit-specific sensitivity of integrin activation

Integrins are α/β heterodimeric transmembrane adhesion receptors. Evidence exists that their transmembrane domain (TMD) separates upon activation. Subunit-specific differences in activation sensitivity of integrins were reported. However, whether sequence variations in the TMD lead to differential TMD association has remained elusive. Here, we show by molecular dynamics simulations and association free energy calculations on TMDs of integrin αIIbβ3, αvβ3, and α5β1 that αIIbβ3 TMD is most stably associated; this difference is related to interaction differences across the TMDs. The order of TMD association stability is paralleled by the basal activity of these integrins, which suggests that TMD differences can have a decisive effect on integrin conformational free energies. We also identified a specific order of clasp disintegration upon TMD dissociation, which suggests that the closed state of integrins may comprise several microstates. Our results provide unprecedented insights into a possibly contributing role of TMD towards subunit-specific sensitivity of integrin activation.

LFA-1 integrin antibodies inhibit leukocyte α4β1-mediated adhesion by intracellular signaling

Binding of intercellular adhesion molecule-1 to the β2-integrin leukocyte function associated antigen-1 (LFA-1) is known to induce cross-talk to the α4β1 integrin. Using different LFA-1 monoclonal antibodies, we have been able to study the requirement and mechanism of action for the cross-talk in considerable detail. LFA-1-activating antibodies and those inhibitory antibodies that signal to α4β1 induce phosphorylation of Thr-758 on the β2-chain, which is followed by binding of 14-3-3 proteins and signaling through the G protein exchange factor Tiam1. This results in dephosphorylation of Thr-788/789 on the β1-chain of α4β1 and loss of binding to its ligand vascular cell adhesion molecule-1. The results show that with LFA-1 antibodies, we can activate LFA-1 and inhibit α4β1, inhibit both LFA-1 and α4β1, inhibit LFA-1 but not α4β1, or not affect LFA-1 or α4β1 These findings are important for the understanding of integrin regulation and for the interpretation of the effect of integrin antibodies and their use in clinical applications.

The integrin αL leg region controls the Mg/EGTA mediated activation of LFA-1

We have shown that Mg/EGTA (5 mM Mg(2+) and 1.5 mM EGTA) could effectively promote the adhesion of integrin αLβ2 to its ligand ICAM-1 but could not promote that of the αMβ2 to denatured BSA. In order to determine the structural differences between αL and αM that specifically contribute to Mg/EGTA sensitivity, a series of αL/αM chimeras were constructed. Our results showed that αLβ2 with αM calf-1 domain completely lost the response to Mg/EGTA activation. In the reverse experiment, αMβ2 would require the presence of both the αL calf-1 and calf-2 domain to initiate the Mg/EGTA sensitivity.

Characterization of single amino acid substitutions in the β2 integrin subunit of patients with leukocyte adhesion deficiency (LAD)-1

Leukocyte adhesion deficiency 1 (LAD-1) is caused by defects in the β2 integrin subunit. We studied 18 missense mutations, 14 of which fail to support the surface expression of the β2 integrins. Integrins with the β2-G150D mutation fail to bind ligands, possibly due to the failure of the α1 segment of the βI domain to assume an α-helical structure. Integrins with the β2-G716A mutation are not maintained in their resting states, and the patient has the severe phenotype of LAD-1. The β2-S453N and β2-P648L mutants support the expression of integrins and adhesion functions. They should be re-classified as polymorphic variants.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Overview: structural biology of integrins

Integrins are cell adhesion molecules that play important roles in many biological processes including hemostasis, immune responses, development, and cancer. Their adhesiveness is dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces outside-in signals from the extracellular domain to the cytoplasm. Advances in the past several years have shed light on structural basis for integrin regulation and signaling, especially how the large-scale reorientations of the ectodomain are related to the inter-domain and intra-domain shape shifting that changes ligand-binding affinity. Experiments have also shown how the conformational changes of the ectodomain are linked to changes in the α- and β-subunit transmembrane and cytoplasmic domains.

Requirement of open headpiece conformation for activation of leukocyte integrin alphaXbeta2

Negative stain electron microscopy (EM) and adhesion assays show that alpha(X)beta(2) integrin activation requires headpiece opening as well as extension. An extension-inducing Fab to the beta(2) leg, in combination with representative activating and inhibitory Fabs, were examined for effect on the equilibrium between the open and closed headpiece conformations. The two activating Fabs stabilized the open headpiece conformation. Conversely, two different inhibitory Fabs stabilized the closed headpiece conformation. Adhesion assays revealed that alpha(X)beta(2) in the extended-open headpiece conformation had high affinity for ligand, whereas both the bent conformation and the extended-closed headpiece conformation represented the low affinity state. Intermediate integrin affinity appears to result not from a single conformational state, but from a mixture of equilibrating conformational states.

Structure of an integrin with an alphaI domain, complement receptor type 4

We report the structure of an integrin with an alphaI domain, alpha(X)beta(2), the complement receptor type 4. It was earlier expected that a fixed orientation between the alphaI domain and the beta-propeller domain in which it is inserted would be required for allosteric signal transmission. However, the alphaI domain is highly flexible, enabling two betaI domain conformational states to couple to three alphaI domain states, and greater accessibility for ligand recognition. Although alpha(X)beta(2) is bent similarly to integrins that lack alphaI domains, the terminal domains of the alpha- and beta-legs, calf-2 and beta-tail, are oriented differently than in alphaI-less integrins. Linkers extending to the transmembrane domains are unstructured. Previous mutations in the beta(2)-tail domain support the importance of extension, rather than a deadbolt, in integrin activation. The locations of further activating mutations and antibody epitopes show the critical role of extension, and conversion from the closed to the open headpiece conformation, in integrin activation. Differences among 10 molecules in crystal lattices provide unprecedented information on interdomain flexibility important for modelling integrin extension and activation.

Permissive transmembrane helix heterodimerization is required for the expression of a functional integrin

The current paradigm is that integrin is activated via inside-out signalling when its cytoplasmic tails and TMs (transmembrane helices) are separated by specific cytosolic protein(s). Perturbations of the helical interface between the α- and β-TMs of an integrin, as a result of mutations, affect its function. Previous studies have shown the requirement for specific pairing between integrin subunits by ectodomain-exchange analyses. It remains unknown whether permissive α/β-TM pairing of an integrin is also required for pairing specificity and the expression of a functionally regulated receptor. We performed scanning replacement of integrin β2-TM with a TM of other integrin β-subunits. With the exception of β4 substitution, others presented β2-integrins with modified phenotypes, either in their expression or ligand-binding properties. Subsequently, we adopted αLβ2 for follow-on experiments because its conformation and affinity-state transitions have been well defined as compared with other members of the β2-integrins. Replacement of β2- with β3-TM generated a chimaeric αLβ2 of an intermediate affinity that adhered to ICAM-1 (intercellular adhesion molecule 1) but not to ICAM-3 constitutively. Replacing αL-TM with αIIb-TM, forming a natural αIIb/β3-TM pair, reversed the phenotype of the chimaera to that of wild-type αLβ2. Interestingly, the replacement of αLβ2- with β3-TM showed neither an extended conformation nor the separation of its cytoplasmic tails, which are well-reported hallmarks of an activated αLβ2, as determined by reporter mAb (monoclonal antibody) KIM127 reactivity and FRET (fluorescence resonance energy transfer) measurements respectively. Collectively, our results suggest that TM pairing specificity is required for the expression of a functionally regulated integrin.

A structural hypothesis for the transition between bent and extended conformations of the leukocyte beta2 integrins

Integrins mediate cell adhesion in response to activation signals that trigger conformational changes within their ectodomain. It is thought that a compact bent conformation of the molecule represents its physiological low affinity state and extended conformations its active state. We have determined the structure of two integrin fragments of the beta2 subunit. The first structure, consisting of the plexin-semaphorin-integrin domain, hybrid, integrin-epidermal growth factor 1 (I-EGF1), and I-EGF2 domains (PHE2), showed an L-shaped conformation with the bend located between the I-EGF1 and I-EGF2 domains. The second structure, which includes, in addition, the I-EGF3 domain, showed an extended conformation. The major reorientation of I-EGF2 with respect to the other domains in the two structures is accompanied by a change of torsion angle of the disulfide bond between Cys(461)-Cys(492) by 180 degrees and the conversion of a short alpha-helix (residues Ser(468)-Cys(475)) into a flexible coil. Based on the PHE2 structure, we introduced a disulfide bond between the plexin-semaphorin-integrin domain and I-EGF2 domains in the beta2 subunit. The resultant alphaLbeta2 integrin (leukocyte function-associated antigen-1) variant was locked in a bent state and could not be detected with the monoclonal antibody KIM127 in Mg(2+)/EGTA. However, it retained the binding activity to ICAM-1. These results provide a structural hypothesis for our understanding of the transition between the resting and active states of leukocyte function-associated antigen-1.

The cytosolic protein talin induces an intermediate affinity integrin alphaLbeta2

The integrin alphaLbeta2 mediates leukocyte adhesion and migration that are required for a functional immune system. It is known that inside-out signaling triggers alphaLbeta2 conformational changes, which affect its ligand-binding affinity. At least three alphaLbeta2 affinity states (low, intermediate, and high) were described. The cytosolic protein talin connects alphaLbeta2 to the actin filament. The talin head domain is also known to activate alphaLbeta2 ligand binding. However, it remains to be determined whether talin promotes an intermediate or high affinity alphaLbeta2. In this study using transfectants and T cells, we showed that talin induced an intermediate affinity alphaLbeta2 that adhered constitutively to its ligand intercellular adhesion molecule (ICAM)-1 but not ICAM-3. Adhesion to ICAM-3 was induced when an additional exogenous activating agent was included. Similar profiles were observed with soluble ICAMs. In addition, the intermediate affinity alphaLbeta2 induced by talin allowed adhesion and migration of T cells on immobilized ICAMs.

Structural basis of integrin regulation and signaling

Integrins are cell adhesion molecules that mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. They play critical roles for the immune system in leukocyte trafficking and migration, immunological synapse formation, costimulation, and phagocytosis. Integrin adhesiveness can be dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces signals from the extracellular domain to the cytoplasm in the classical outside-in direction. Recent structural, biochemical, and biophysical studies have greatly advanced our understanding of the mechanisms of integrin bidirectional signaling across the plasma membrane. Large-scale reorientations of the ectodomain of up to 200 A couple to conformational change in ligand-binding sites and are linked to changes in alpha and beta subunit transmembrane domain association. In this review, we focus on integrin structure as it relates to affinity modulation, ligand binding, outside-in signaling, and cell surface distribution dynamics.

Mutation of a conserved asparagine in the I-like domain promotes constitutively active integrins alphaLbeta2 and alphaIIbbeta3

The leukocyte beta2 integrins are heterodimeric adhesion receptors required for a functional immune system. Many leukocyte adhesion deficiency-1 (LAD-1) mutations disrupt the expression and function of beta2 integrins. Herein, we further characterized the LAD-1 mutation N329S in the beta2 inserted (I)-like domain. This mutation converted alphaLbeta2 from a resting into a high affinity conformer because alphaLbeta2N329S transfectants adhered avidly to ligand intercellular adhesion molecule (ICAM)-3 in the absence of additional activating agent. An extended open conformation is adopted by alphaLbeta2N329S because of its reactivity with the beta2 activation reporter monoclonal antibodies MEM148 and KIM127. A corresponding mutation in beta3 generated constitutively active alphaIIbbeta3 that adhered to fibrinogen. This Asn is conserved in all human beta subunits, and it resides before the last helix of the I-like domain, which is known to be important in activation signal propagation. By mutagenesis studies and review of existing integrin structures, we conjectured that this conserved Asn may have a primary role in shaping the I-like domain by stabilizing the conformation of the alpha7 helix and the beta6-alpha7 loop in the I-like domain.

Tests of the extension and deadbolt models of integrin activation

Despite extensive evidence that integrin conformational changes between bent and extended conformations regulate affinity for ligands, an alternative hypothesis has been proposed in which a "deadbolt" can regulate affinity for ligand in the absence of extension. Here, we tested both the deadbolt and the extension models. According to the deadbolt model, a hairpin loop in the beta3 tail domain could act as a deadbolt to restrain the displacement of the beta3 I domain beta6-alpha7 loop and maintain integrin in the low affinity state. We found that mutating or deleting the beta3 tail domain loop has no effect on ligand binding by either alphaIIbbeta 3 or alphaVbeta3 integrins. In contrast, we found that mutations that lock integrins in the bent conformation with disulfide bonds resist inside-out activation induced by cytoplasmic domain mutation. Furthermore, we demonstrated that extension is required for accessibility to fibronectin but not smaller fragments. The data demonstrate that integrin extension is required for ligand binding during integrin inside-out signaling and that the deadbolt does not regulate integrin activation.

Integrin structures and conformational signaling

Integrins are cell adhesion molecules that play critical roles in development, wound healing, hemostasis, immunity and cancer. Advances in the past two years have shed light on the structural basis for integrin regulation and signaling, especially on how global conformational changes between bent and extended conformations relate to the inter-domain and intra-domain shape shifting that regulates affinity for ligand. The downward movements of the C-terminal helices of the alpha I and beta I domains and the swing-out of the hybrid domain play pivotal roles in integrin conformational signaling. Experiments have also shown that integrins transmit bidirectional signals across the plasma membrane by coupling extracellular conformational change with an unclasping and separation of the alpha and beta transmembrane and cytoplasmic domains.

Down-regulation of integrin alpha M beta 2 ligand-binding function by the urokinase-type plasminogen activator receptor

The cell adhesion molecule integrin alphaMbeta2 associates with the urokinase-type plasminogen activator receptor (uPAR) on monocytes and neutrophils. uPAR also associates with members of the beta1 and beta3 integrins, and it modulates the ligand-binding function of these integrins. In this study, we showed that co-expressing uPAR with alphaMbeta2 in 293 transfectants down-regulated the ligand-binding capacity of alphaMbeta2 to denatured protein, fibrinogen, and intercellular adhesion molecule 1 (ICAM-1). Migration of transfectants on fibrinogen mediated by alphaMbeta2 was reduced in the presence of uPAR. In addition, the constitutive ligand-binding property of an alphaMbeta2 mutant was attenuated by its association with uPAR. Co-immunoprecipitation analyses using a panel of alphaMbeta2-specific mAbs suggest shielding of the ligand-recognition site of alphaMbeta2 by uPAR.

Integrin structure, allostery, and bidirectional signaling

Alphabeta heterodimeric integrins mediate dynamic adhesive cell-cell and cell-extracellular matrix (ECM) interactions in metazoa that are critical in growth and development, hemostasis, and host defense. A central feature of these receptors is their capacity to change rapidly and reversibly their adhesive functions by modulating their ligand-binding affinity. This is normally achieved through interactions of the short cytoplasmic integrin tails with intracellular proteins, which trigger restructuring of the ligand-binding site through long-range conformational changes in the ectodomain. Ligand binding in turn elicits conformational changes that are transmitted back to the cell to regulate diverse responses. The publication of the integrin alphaVbeta3 crystal structure has provided the context for interpreting decades-old biochemical studies. Newer NMR, crystallographic, and EM data, reviewed here, are providing a better picture of the dynamic integrin structure and the allosteric changes that guide its diverse functions.

Two Functional Active Conformations of the Integrin α2β1, Depending on Activation Condition and Cell Type

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin alpha2beta1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of alpha2beta1 because it only binds to the activated state of alpha2beta1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of alpha2beta1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of alpha2beta1 are dependent on the cell type where alpha2beta1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and alpha2beta1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of alpha2beta1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells.

Epitope mapping of monoclonal antibody to integrin alphaL beta2 hybrid domain suggests different requirements of affinity states for intercellular adhesion molecules (ICAM)-1 and ICAM-3 binding

Integrin undergoes different activation states by changing its quaternary conformation. The integrin beta hybrid domain acts as a lever for the transmission of activation signal. The displacement of the hybrid domain can serve to report different integrin activation states. The monoclonal antibody (mAb) MEM148 is a reporter antibody that recognizes Mg/EGTA-activated but not resting integrin alpha(L) beta2. Herein, we mapped its epitope to the critical residue Pro374 located on the inner face of the beta2 hybrid domain. Integrin alpha(L) beta2 binds to its ligands ICAM-1 and ICAM-3 with different affinities. Integrin is proposed to have at least three affinity states, and the position of the hybrid domain differs in each. We made use of the property of mAb MEM148 to analyze and correlate these affinity states in regard to alpha(L) beta2/intercellular adhesion molecule (ICAM) binding. Our study showed that Mg/EGTA-activated alpha(L)beta2 can adopt a different conformation from that activated by activating mAbs KIM185 or MEM48. Unlike ICAM-1 binding, which required only one activating agent, alpha(L) beta2/ICAM-3 binding required both Mg/EGTA and an activating mAb. This suggests that alpha(L)beta2 with intermediate affinity is sufficient to bind ICAM-1 but not ICAM-3, which requires a high affinity state. Furthermore, we showed that the conformation adopted by alpha(L)beta2 in the presence of Mg/EGTA, depicting an intermediate activation state, could be reverted to its resting conformation.