Ligand binding promotes the entropy-driven oligomerization of integrin alpha IIb beta 3

Identification of Sca-1+Abcg1+ bronchioalveolar epithelial cells as the origin of lung adenocarcinoma in Gprc5a-knockout mouse model through the interaction between lung progenitor AT2 and Lgr5 cells

The reason for the reduced efficacy of lung cancer therapy is the existence of lung cancer stem cells (CSCs). Targeting CSCs results in evolved phenotypes with increased malignancy, leading to therapy failure. Here, we propose a new therapeutic strategy: investigating the “transitional” cells that represent the stage between normal lung stem cells and lung CSCs. Identifying and targeting the key molecule that drives carcinogenesis to inhibit or reverse this process would thus provide new perspectives for early diagnosis and intervention in lung cancer. We used Gprc5a-knockout (KO) mice, the first animal model of spontaneous lung adenocarcinoma established by the deletion of a single lung tumor suppressor gene. We investigated the interaction of lung progenitor cells AT2 with Lgr5 cells in the generation of CSCs and related signaling mechanism. In the present study, using Gprc5a-KO mice, we found the initiator Sca-1⁺Abcg1⁺ subset with a CSC-like phenotype within the lung progenitor AT2 cell population in mice that had not yet developed tumors. We confirmed the self-renewal and tumor initiation capacities of this subset in vitro, in vivo, and clinical samples. Mechanistically, we found that the generation of Sca-1⁺Abcg1⁺ cells was associated with an interaction between AT2 and Lgr5 cells and the subsequent activation of the ECM1-α6β4-ABCG1 axis. Importantly, Sca-1⁺Abcg1⁺ and SPA⁺ABCG1⁺ cells specifically existed in the small bronchioles of Gprc5a-KO mice and patients with pneumonia, respectively. Thus, the present study unveiled a new kind of lung cancer-initiating cells (LCICs) and provided potential markers for the early diagnosis of lung cancer.

The lateral diffusion and fibrinogen induced clustering of platelet integrin αIIbβ3 reconstituted into physiologically mimetic GUVs

Platelet integrin αIIbβ3 is a key mediator of platelet activation and thrombosis. Upon activation αIIbβ3 undergoes significant conformational rearrangement, inducing complex bidirectional signalling and protein recruitment leading to platelet activation. Reconstituted lipid models of the integrin can enhance our understanding of the structural and mechanistic details of αIIbβ3 behaviour away from the complexity of the platelet machinery. Here, a novel method of αIIbβ3 insertion into Giant Unilamellar Vesicles (GUVs) is described that allows for effective integrin reconstitution unrestricted by lipid composition. αIIbβ3 was inserted into two GUV lipid compositions that seek to better mimic the platelet membrane. First, "nature's own", comprising 32% DOPC, 25% DOPE, 20% CH, 15% SM and 8% DOPS, intended to mimic the platelet cell membrane. Fluorescence Lifetime Correlation Spectroscopy (FLCS) reveals that exposure of the integrin to the activators Mn(2+) or DTT does not influence the diffusion coefficient of αIIbβ3. Similarly, exposure to αIIbβ3's primary ligand fibrinogen (Fg) alone does not affect αIIbβ3's diffusion coefficient. However, addition of Fg with either activator reduces the integrin diffusion coefficient from 2.52 ± 0.29 to μm(2) s(-1) to 1.56 ± 0.26 (Mn(2+)) or 1.49 ± 0.41 μm(2) s(-1) (DTT) which is consistent with aggregation of activated αIIbβ3 induced by fibrinogen binding. The Multichannel Scaler (MCS) trace shows that the integrin-Fg complex diffuses through the confocal volume in clusters. Using the Saffman-Delbrück model as a first approximation, the diffusion coefficient of the complex suggests at least a 20-fold increase in the radius of membrane bound protein, consistent with integrin clustering. Second, αIIbβ3 was also reconstituted into a "raft forming" GUV with well defined liquid disordered (Ld) and liquid ordered (Lo) phases. Using confocal microscopy and lipid partitioning dyes, αIIbβ3 showed an affinity for the DOPC rich Ld phase of the raft forming GUVs, and was effectively excluded from the cholesterol and sphingomyelin rich Lo phase. Activation and Fg binding of the integrin did not alter the distribution of αIIbβ3 between the lipid phases. This observation suggests partitioning of the activated fibrinogen bound αIIbβ3 into cholesterol rich domains is not responsible for the integrin clustering observed.

Platelet Activation and Thrombus Formation over IgG Immune Complexes Requires Integrin αIIbβ3 and Lyn Kinase

IgG immune complexes contribute to the etiology and pathogenesis of numerous autoimmune disorders, including heparin-induced thrombocytopenia, systemic lupus erythematosus, rheumatoid- and collagen-induced arthritis, and chronic glomerulonephritis. Patients suffering from immune complex-related disorders are known to be susceptible to platelet-mediated thrombotic events. Though the role of the Fc receptor, FcγRIIa, in initiating platelet activation is well understood, the role of the major platelet adhesion receptor, integrin αIIbβ3, in amplifying platelet activation and mediating adhesion and aggregation downstream of encountering IgG immune complexes is poorly understood. The goal of this investigation was to gain a better understanding of the relative roles of these two receptor systems in immune complex-mediated thrombotic complications. Human platelets, and mouse platelets genetically engineered to differentially express FcγRIIa and αIIbβ3, were allowed to interact with IgG-coated surfaces under both static and flow conditions, and their ability to spread and form thrombi evaluated in the presence and absence of clinically-used fibrinogen receptor antagonists. Although binding of IgG immune complexes to FcγRIIa was sufficient for platelet adhesion and initial signal transduction events, platelet spreading and thrombus formation over IgG-coated surfaces showed an absolute requirement for αIIbβ3 and its ligands. Tyrosine kinases Lyn and Syk were found to play key roles in IgG-induced platelet activation events. Taken together, our data suggest a complex functional interplay between FcγRIIa, Lyn, and αIIbβ3 in immune complex-induced platelet activation. Future studies may be warranted to determine whether patients suffering from immune complex disorders might benefit from treatment with anti-αIIbβ3-directed therapeutics.

An optics-based variable-temperature assay system for characterizing thermodynamics of biomolecular reactions on solid support

A biological state is equilibrium of multiple concurrent biomolecular reactions. The relative importance of these reactions depends on physiological temperature typically between 10 °C and 50 °C. Experimentally the temperature dependence of binding reaction constants reveals thermodynamics and thus details of these biomolecular processes. We developed a variable-temperature opto-fluidic system for real-time measurement of multiple (400-10,000) biomolecular binding reactions on solid supports from 10 °C to 60 °C within ±0.1 °C. We illustrate the performance of this system with investigation of binding reactions of plant lectins (carbohydrate-binding proteins) with 24 synthetic glycans (i.e., carbohydrates). We found that the lectin-glycan reactions in general can be enthalpy-driven, entropy-driven, or both, and water molecules play critical roles in the thermodynamics of these reactions.

P-selectin glycoprotein ligand-1 forms dimeric interactions with E-selectin but monomeric interactions with L-selectin on cell surfaces

Interactions of selectins with cell surface glycoconjugates mediate the first step of the adhesion and signaling cascade that recruits circulating leukocytes to sites of infection or injury. P-selectin dimerizes on the surface of endothelial cells and forms dimeric bonds with P-selectin glycoprotein ligand-1 (PSGL-1), a homodimeric sialomucin on leukocytes. It is not known whether leukocyte L-selectin or endothelial cell E-selectin are monomeric or oligomeric. Here we used the micropipette technique to analyze two-dimensional binding of monomeric or dimeric L- and E-selectin with monomeric or dimeric PSGL-1. Adhesion frequency analysis demonstrated that E-selectin on human aortic endothelial cells supported dimeric interactions with dimeric PSGL-1 and monomeric interactions with monomeric PSGL-1. In contrast, L-selectin on human neutrophils supported monomeric interactions with dimeric or monomeric PSGL-1. Our work provides a new method to analyze oligomeric cross-junctional molecular binding at the interface of two interacting cells.

PROBING αIIbβ3: LIGAND INTERACTIONS BY DYNAMIC FORCE SPECTROSCOPY AND SURFACE PLASMON RESONANCE

The interaction between platelet integrin αIIbβ3 and fibrin(ogen) plays a key role in blood clot formation and stability. Integrin antagonists, a class of pharmaceuticals used to prevent and treat cardiovascular disease, are designed to competitively interfere with this process. However, the energetics of the integrin-drug binding are not fully understood, potentially hampering further development of this class of pharmaceuticals. We integrated dynamic force spectroscopy (DFS) and surface plasmon resonance (SPR) to probe the energetics of complex formation between αIIbβ3 and cHarGD, a cyclic peptide integrin antagonist. Analysis of αIIbβ3:cHarGD DFS rupture force data at pulling rates of 14 000 pN/s, 42 000 pN/s and 70 000 pN/s yielded koff = 0.02-0.09 s-1, a dissociation energy barrier [Formula: see text] = 22-29 kJ/mol, and a potential well width x-1 = 0.5-0.8 nm. SPR kinetic data yielded an association rate constant kon = 7 × 103 L/mol-s and a dissociation rate constant koff = 10-2 s-1, followed by a slower stabilization step (τ ~ 400 s). Both DFS and SPR detected minimal interactions between αIIbβ3 and cHarGA demonstrating a key role for electrostatic interactions between the ligand aspartate and the integrin metal ion-dependent adhesion site (MIDAS). Our work provides new insights into the energy landscape of αIIbβ3's interactions with pharmacological and physiological ligands.

Transmembrane domains interactions within the membrane milieu: principles, advances and challenges

Protein-protein interactions within the membrane are involved in many vital cellular processes. Consequently, deficient oligomerization is associated with known diseases. The interactions can be partially or fully mediated by transmembrane domains (TMD). However, in contrast to soluble regions, our knowledge of the factors that control oligomerization and recognition between the membrane-embedded domains is very limited. Due to the unique chemical and physical properties of the membrane environment, rules that apply to interactions between soluble segments are not necessarily valid within the membrane. This review summarizes our knowledge on the sequences mediating TMD-TMD interactions which include conserved motifs such as the GxxxG, QxxS, glycine and leucine zippers, and others. The review discusses the specific role of polar, charged and aromatic amino acids in the interface of the interacting TMD helices. Strategies to determine the strength, dynamics and specificities of these interactions by experimental (ToxR, TOXCAT, GALLEX and FRET) or various computational approaches (molecular dynamic simulation and bioinformatics) are summarized. Importantly, the contribution of the membrane environment to the TMD-TMD interaction is also presented. Studies utilizing exogenously added TMD peptides have been shown to influence in vivo the dimerization of intact membrane proteins involved in various diseases. The chirality independent TMD-TMD interactions allows for the design of novel short d- and l-amino acids containing TMD peptides with advanced properties. Overall these studies shed light on the role of specific amino acids in mediating the assembly of the TMDs within the membrane environment and their contribution to protein function. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Adhesive forces in embryonic stem cell cultures

Most cell culture systems grow and spread as contact-inhibited monolayers on flat culture dishes, but the embryonic stem cell (ESC) is one of the cell phenotypes that prefer to self-organize as tightly packed three-dimensional (3D) colonies. ESC also readily form 3D cell aggregates, called embryoid bodies (EB) that partially mimic the spatial and temporal processes of the developing embryo. Here, the rationale for ESC aggregatation, rather than "spreading" on gelatin-coated or mouse embryonic fibroblast (MEF)-coated dishes, is examined through the quantification of the expression levels of adhesion molecules on ESC and the calculation of the adhesive forces on ESC. Modeling each ESC as a dodecahedron, the adhesive force for each ESC-ESC binding was found to be 9.1 x 10(5) pN, whereas, the adhesive force for ESC-MEF binding was found to be an order of magnitude smaller at 7.9 x 10(4) pN. We also show that E-cadherin is the dominating molecule in the ESC-ESC adhesion and blocking E-cadherin leads to a significant reduction in colony formation. Here, we mathematically describe the preference for ESC to self-assemble into ESC-ESC aggregates and 3D colonies, rather than to bind and spread on gelatin or MEF-coated dishes, and have shown that these interactions are predominantly due to E-cadherin expression on ESC.

Dynamic regulation of fibrinogen: integrin αIIbβ3 binding

This study demonstrates that two orthogonal events regulate integrin αIIbβ3's interactions with fibrinogen, its primary physiological ligand: (1) conformational changes at the αIIb-β3 interface and (2) flexibility in the carboxy terminus of fibrinogen's γ-module. The first postulate was tested by capturing αIIbβ3 on a biosensor and measuring binding by surface plasmon resonance. Binding of fibrinogen to eptifibatide-primed αIIbβ3 was characterized by a k(on) of ~2 × 10(4) L mol(-1) s(-1) and a k(off) of ~8 × 10(-5) s(-1) at 37 °C. In contrast, even at 150 nM fibrinogen, no binding was detected with resting αIIbβ3. Eptifibatide competitively inhibited fibrinogen's interactions with primed αIIbβ3 (K(i) ~0.4 nM), while a synthetic γ-module peptide (HHLGGAKQAGDV) was only weakly inhibitory (K(i) > 10 μM). The second postulate was tested by measuring αIIbβ3's interactions with recombinant fibrinogen, both normal (rFgn) and a deletion mutant lacking the γ-chain AGDV sites (rFgn γΔ408-411). Normal rFgn bound rapidly, tightly, and specifically to primed αIIbβ3; no interaction was detected with rFgn γΔ408-411. Equilibrium and transition-state thermodynamic data indicated that binding of fibrinogen to primed αIIbβ3, while enthalpy-favorable, must overcome an entropy-dominated activation energy barrier. The hypothesis that fibrinogen binding is enthalpy-driven fits with structural data showing that its γ-C peptide and eptifibatide exhibit comparable electrostatic contacts with αIIbβ3's ectodomain. The concept that fibrinogen's αIIbβ3 targeting sequence is intrinsically disordered may explain the entropy penalty that limits its binding rate. In the hemostatic milieu, platelet-platelet interactions may be localized to vascular injury sites because integrins must be activated before they can bind their most abundant ligand.

Tests of integrin transmembrane domain homo-oligomerization during integrin ligand binding and signaling

Integrin transmembrane (TM) and/or cytoplasmic domains play a critical role in integrin bidirectional signaling. Although it has been shown that TM and/or cytoplasmic α and β domains associate in the resting state and separation of these domains is required for both inside-out and outside-in signaling, the role of TM homomeric association remains elusive. Formation of TM homo-oligomers was observed in micelles and bacterial membranes previously, and it has been proposed that homomeric association is important for integrin activation and clustering. This study addresses whether integrin TM domains form homo-oligomers in mammalian cell membranes using cysteine scanning mutagenesis. Our results show that TM homomeric interaction does not occur before or after soluble ligand binding or during inside-out activation. In addition, even though the cysteine mutants and the heterodimeric disulfide-bounded mutant could form clusters after adhering to immobilized ligand, the integrin TM domains do not form homo-oligomers, suggesting that integrin TM homomeric association is not critical for integrin clustering or outside-in signaling. Therefore, integrin TM homo-oligomerization is not required for integrin activation, ligand binding, or signaling.

Shedding of large functionally active CD11/CD18 Integrin complexes from leukocyte membranes during synovial inflammation distinguishes three types of arthritis through differential epitope exposure

CD18 integrins are adhesion molecules expressed on the cell surface of leukocytes and play a central role in the molecular mechanisms supporting leukocyte migration to zones of inflammation. Recently, it was discovered that CD11a/CD18 is shed from the leukocyte surface in models of inflammation. In this study, we show that shedding of human CD11/CD18 complexes is a part of synovial inflammation in rheumatoid arthritis and spondyloarthritis but not in osteoarthritis. In vivo and in vitro data suggest that the shedding is driven by TNF-α, which links the process to central events in the inflammatory response. The shed complexes contain multiple heterodimers of CD11/CD18, are variable in size, and differ according to the type of synovial inflammation. Furthermore, the differential structures determine the avidity of binding of the complexes to the ICAM-1. With the estimated concentrations of CD11/CD18 in plasma and synovial fluid a significant coverage of binding sites in ICAM-1 for CD18 integrins is expected. Based on cell adhesion experiments in vitro, we hypothesize that the large soluble complexes of CD11/CD18 act in vivo to buffer leukocyte adhesion by competing with the membrane-bound receptors for ICAM-1 binding sites. As reported here for synovial inflammation changes in the concentration or structure of these complexes should be considered as likely contributors to disease activity.

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.

Integrin conformational regulation: uncoupling extension/tail separation from changes in the head region by a multiresolution approach

Integrin-dependent adhesion and signaling are regulated by conformational changes whose details remain controversial. Crystallography revealed bent shapes for resting and primed integrin ectodomains, whereas large, ligand-induced rearrangements in other constructs suggested extension, "opening," and tail separation. We have used experimental/computed hydrodynamics to discriminate among different alpha(v)beta(3) and alpha(IIb)beta(3) atomic models built on X-ray, NMR, and EM data. In contrast with X-ray structures and EM maps, hydrodynamics indicate that resting integrins are already extended. Furthermore, the hydrodynamics of an alpha(v)beta(3) ectodomain-fibronectin fragment complex support opening via additional head region conformational changes (hybrid domain swing-out), but without tail separation. Likewise, frictional changes induced by priming agents in full-length alpha(IIb)beta(3) correlate well with the swing-out coupled to a simple transmembrane helix shift in an extended, electron tomography-based model. Extension and immediate tail separation are then uncoupled from head region rearrangements following activation, thus underscoring integrins' delicate, finely tuned plasticity.

NMR solution structure, stability, and interaction of the recombinant bovine fibrinogen alphaC-domain fragment

According to the existing hypothesis, in fibrinogen, the COOH-terminal portions of two Aalpha chains are folded into compact alphaC-domains that interact intramolecularly with each other and with the central region of the molecule; in fibrin, the alphaC-domains switch to an intermolecular interaction resulting in alphaC-polymers. In agreement, our recent NMR study identified within the bovine fibrinogen Aalpha374-538 alphaC-domain fragment an ordered compact structure including a beta-hairpin restricted at the base by a 423-453 disulfide linkage. To establish the complete structure of the alphaC-domain and to further test the hypothesis, we expressed a shorter alphaC-fragment, Aalpha406-483, and performed detailed analysis of its structure, stability, and interactions. NMR experiments on the Aalpha406-483 fragment identified a second loose beta-hairpin formed by residues 459-476, yielding a structure consisting of an intrinsically unstable mixed parallel/antiparallel beta-sheet. Size-exclusion chromatography and sedimentation velocity experiments revealed that the Aalpha406-483 fragment forms soluble oligomers whose fraction increases with an increase in concentration. This was confirmed by sedimentation equilibrium analysis, which also revealed that the addition of each monomer to an assembling alphaC-oligomer substantially increases its stabilizing free energy. In agreement, unfolding experiments monitored by CD established that oligomerization of Aalpha406-483 results in increased thermal stability. Altogether, these experiments establish the complete NMR solution structure of the Aalpha406-483 alphaC-domain fragment, provide direct evidence for the intra- and intermolecular interactions between the alphaC-domains, and confirm that these interactions are thermodynamically driven.

AlphaIIbbeta3 priming and clustering by orally active and intravenous integrin antagonists

BACKGROUND

Drugs that block platelet-platelet and platelet-fibrin interactions via the alpha(IIb)beta(3) (glycoprotein IIb/IIIa) receptor are used daily in patients undergoing percutaneous coronary interventions. Along with expected increases in spontaneous bleeding, clinical trials have revealed a surprising increase in thrombosis when these drugs are used without other anticoagulants. A better understanding of their mechanisms can minimize these risks.

OBJECTIVES

This study tested the hypothesis that interventions designed to block fibrinogen binding inevitably leave the alpha(IIb)beta(3) receptor in an activated state. It compared the effects on platelet function and alpha(IIb)beta(3) conformation of the orally active compounds orbofiban and roxifiban, the i.v. agents eptifibatide and tirofiban, and echistatin, an arginine-glycine-aspartate (RGD) disintegrin.

METHODS

The integrin antagonist concentrations required to saturate platelets and to block platelet-platelet and platelet-fibrin interactions were determined by flow cytometry, aggregometry, and clot-based adhesion assays, respectively. Analytical ultracentrifugation measured each antagonist's effects on the solution structure of alpha(IIb)beta(3). Fluorescence anisotropy provided equilibrium and kinetic data for integrin:antagonist interactions.

RESULTS

Both orally active drugs bound more tightly and inhibited platelet aggregation and adhesion to fibrin more effectively than echistatin. Analytical ultracentrifugation yielded this order for perturbing alpha(IIb)beta(3) conformation (priming) and promoting oligomerization (clustering): echistatin > eptifibatide > orbofiban > tirofiban > roxifiban. Roxifiban was also most effective at disrupting the rapidly forming/slowly dissociating alpha(IIb)beta(3):echistatin complex.

CONCLUSIONS

Our results suggest that the same molecular mechanisms that enable glycoprotein IIb/IIIa inhibitors to bind tightly to the alpha(IIb)beta(3) receptor and block fibrinogen binding contribute to their ability to perturb the resting integrin's conformation, thus limiting the safety and efficacy of both oral and i.v. integrin antagonists.

Purification, analysis, and crystal structure of integrins

Integrins are large modular cell-surface receptors that regulate almost every aspect of cellular function through bidirectional signals transmitted across the lipid bilayer. Regulation of integrin activity is accomplished by complex and still incompletely understood biochemical pathways that modify integrin ligand binding, clustering, trafficking, and signaling functions. The dynamic tertiary and quaternary changes required to channel some of these activities have hampered, until recently, the crystal structure determination of these heterodimeric receptors. In this chapter, we review the methods used to purify and characterize these proteins biophysically and functionally, and to derive their three-dimensional structures.

Integrin alphaIIbbeta3:ligand interactions are linked to binding-site remodeling

This study tested the hypothesis that high-affinity binding of macromolecular ligands to the alphaIIbbeta3 integrin is tightly coupled to binding-site remodeling, an induced-fit process that shifts a conformational equilibrium from a resting toward an open receptor. Interactions between alphaIIbbeta3 and two model ligands-echistatin, a 6-kDa recombinant protein with an RGD integrin-targeting sequence, and fibrinogen's gamma-module, a 30-kDa recombinant protein with a KQAGDV integrin binding site-were measured by sedimentation velocity, fluorescence anisotropy, and a solid-phase binding assay, and modeled by molecular graphics. Studying echistatin variants (R24A, R24K, D26A, D26E, D27W, D27F), we found that electrostatic contacts with charged residues at the alphaIIb/beta3 interface, rather than nonpolar contacts, perturb the conformation of the resting integrin. Aspartate 26, which interacts with the nearby MIDAS cation, was essential for binding, as D26A and D26E were inactive. In contrast, R24K was fully and R24A partly active, indicating that the positively charged arginine 24 contributes to, but is not required for, integrin recognition. Moreover, we demonstrated that priming--i.e., ectodomain conformational changes and oligomerization induced by incubation at 35 degrees C with the ligand-mimetic peptide cHarGD--promotes complex formation with fibrinogen's gamma-module. We also observed that the gamma-module's flexible carboxy terminus was not required for alphaIIbbeta3 integrin binding. Our studies differentiate priming ligands, which bind to the resting receptor and perturb its conformation, from regulated ligands, where binding-site remodeling must first occur. Echistatin's binding energy is sufficient to rearrange the subunit interface, but regulated ligands like fibrinogen must rely on priming to overcome conformational barriers.

The Identification of a Minimal Dimerization Motif QXXS That Enables Homo- and Hetero-association of Transmembrane Helices in Vivo

Assembly of transmembrane (TM) domains is a critical step in the function of membrane proteins, and therefore, determining the amino acid motifs that mediate this process is important. Studies along this line have shown that the GXXXG motif is involved in TM assembly. In this study we characterized the minimal dimerization motif in the bacterial Tar-1 homodimer TM domain, which does not contain a GXXXG sequence. We found that a short polar motif QXXS is sufficient to induce stable TM-TM interactions. Statistical analysis revealed that this motif appears to be significantly over-represented in a bacterial TM data base compared with its theoretical expectancy, suggesting a general role for this motif in TM assembly. A truncated short TM peptide (9 residues) that contains the QXXS motif interacted slightly with the wild-type Tar-1. However, the same short TM peptide regained wild-type-like activity when conjugated to an octanoyl aliphatic moiety. Biophysical studies indicated that this modification compensated for the missing hydrophobicity, stabilized alpha-helical structure, and enabled insertion of the peptide into the membrane core. These findings serve as direct evidence that even a short peptide containing a minimal recognition motif is sufficient to inhibit the proper assembly of TM domains. Interestingly, electron microscopy revealed that above the critical micellar concentration, the TM lipopeptide forms a network of nanofibers, which can serve for the slow release of the active lipopeptide.

Functional and structural correlations of individual αIIbβ3 molecules

The divalent cation Mn2+ and the reducing agent dithiothreitol directly shift integrins from their inactive to their active states. We used transmission electron microscopy and laser tweezers-based force spectroscopy to determine whether structural rearrangements induced by these agents in the integrin αIIbβ3 correlate with its ability to bind fibrinogen. Mn2+ increased the probability of specific fibrinogen-αIIbβ3 interactions nearly 20-fold in platelets, and both Mn2+ and dithiothreitol increased the probability more than 2-fold using purified proteins. Of 3 αIIbβ3 conformations, closed with stalks touching, open with stalks separated, and globular without visible stalks, Mn2+ and dithiothreitol induced a significant increase in the proportion of open structures, as well as structural changes in the αIIbβ3 headpiece. Mn2+ also increased the number of complexes between fibrinogen and purified αIIbβ3 molecules, all of which were in the open conformation. Finally, Mn2+ induced the formation of αIIbβ3 clusters that resulted from interactions exclusively involving the distal ends of the stalks. These results indicate that there is a direct correlation between αIIbβ3 activation and the overall conformation of the molecule. Further, they are consistent with the presence of a linked equilibrium between single inactive and single active αIIbβ3 molecules and active αIIbβ3 clusters. (Blood. 2004;104:3979-3985)

The disintegrin echistatin stabilizes integrin alphaIIbbeta3's open conformation and promotes its oligomerization

We have employed echistatin, a 5.4 kDa snake venom disintegrin, as a model protein to investigate the paradox that small ligand-mimetics can bind to the resting alphaIIbbeta3 integrin while adhesive macromolecules cannot. We characterized the interactions between purified human alphaIIbbeta3 and two recombinant echistatin variants: rEch (1-49) M28L, chosen for its selectivity toward beta3-integrins, and rEch (1-40) M28L, a carboxy-terminal truncation mutant. While both contain an RGD integrin targeting sequence, only rEch (1-49) M28L was an effective inhibitor of alphaIIbbeta3 function. Electron microscopy of rotary shadowed specimens yielded a variety of alphaIIbbeta3 conformers ranging from compact, spherical particles (maximum dimension 22 nm) to the classical "head with two tails" forms (32 nm). The population of larger particles (42-56 nm) increased from 17% to 28% in the presence of rEch (1-49) M28L, indicative of ligand-induced oligomerization. Sedimentation velocity measurements demonstrated that both full length and truncated echistatin perturbed alphaIIbbeta3's solution structure, yielding slower-sedimenting open conformers. Dynamic light scattering showed that rEch (1-49) M28L protected alphaIIbbeta3 from thermal aggregation, raising its transition mid-point from 46 degrees C to 69 degrees C; a smaller shift resulted with rEch (1-40) M28L. Sedimentation equilibrium demonstrated that both echistatin ligands induced substantial alphaIIbbeta3 dimerization. van't Hoff analysis revealed a pattern of entropy/enthalpy compensation similar to tirofiban, a small RGD ligand-mimetic that binds tightly to alphaIIbbeta3, but yields smaller conformational perturbations than echistatin. We propose that echistatin may serve as a paradigm for understanding multidomain adhesive macromolecules because its ability to modulate alphaIIbbeta3's structure resides on an RGD loop, while full disintegrin activity requires an auxiliary site that includes the carboxy-terminal nine amino acid residues.

Structures of integrin domains and concerted conformational changes in the bidirectional signaling mechanism of alphaIIbbeta3

Integrins are heterodimeric type I transmembrane cell-adhesive receptors whose affinity for ligands is regulated by tertiary and quaternary conformational changes that are transmitted from the cytoplasmic tails to the extracellular ectodomains during the transition from the inactive to the active state. Receptor occupancy initiates further structural alterations that transduce signals across the plasma membrane and result in receptor clustering and recruitment of signaling molecules and cytoskeletal rearrangements at the integrin's cytoplasmic domains. The large distance between the intracellular cytoplasmic domains and the ligand-binding site, which in an extended conformation spans more that 200 A, imposes a complex mechanism of interdomain communication for the bidirectional information flow across the plasma membrane. Significant progress has recently been made in elucidating the crystal and electron microscopy structures of integrin ectodomains in its unliganded and liganded states, and the nuclear magnetic resonance solution structures of stalk domains and the cytoplasmic tails. These structures revealed the location of sites that are functionally important and provided the basis for defining new models of integrin activation and signaling through bidirectional conformational changes, and for understanding the structural basis of the cation-dependent ligand-binding specificity of integrins. Platelet integrin alphaIIbbeta3 has served as a paradigm for many aspects of the structure and function of integrins The aim of this minireview is to combine recent structural and biochemical studies on integrin receptors that converge into a model of the tertiary and quaternary conformational changes in alphaIIbbeta3 and other homologous integrins that propagate inside-out and outside-in signals.

The platelet integrin alpha(IIb) beta(3) imaged by atomic force microscopy on model surfaces

The platelet membrane receptor alpha(IIb) beta(3) binds to adsorbed protein ligands including fibrinogen, von Willebrand factor and fibronectin, and is critically important in mediating platelet adhesion to damaged subendothelium and to synthetic biomaterial surfaces. This receptor is a member of the integrin family, a highly prevalent class of heterodimeric molecules consisting of a single alpha and beta subunit. In an ongoing effort to understand the mechanisms underlying platelet adhesion events, high-resolution atomic force microscopy (AFM) under dynamic conditions was used to obtain images of alpha(IIb) beta(3) molecules as well as aggregates of the protein. Images of integrin molecules were obtained by tapping mode AFM under aqueous buffer conditions following adsorption on a series of ultrasmooth model surfaces. On a model hydrophobic surface, detergents stabilizing the protein in solution competed for surface adsorption sites. When this detergent was removed from the system, the protein was predominantly seen as aggregates with head groups pointing outward. A limited number of individual integrin molecules were observed, and were found to have dimensions consistent with those reported previously by electron microscopy studies. Integrin molecules showed weak adhesion to the two hydrophilic surfaces used in the study, although formation of a lipid bilayer around surface-adsorbed molecules improved the resolution. At longer time periods, the integrin molecules embedded in this lipid bilayer exhibited sufficient mobility to form molecular aggregates. The structural measurements described in this study not only reveal three-dimensional features of the molecule, they represent an important step towards dynamic adsorption experiments and visualizing the integrin interacting with surface-adsorbed proteins as in biomaterial-induced thrombogenesis.

Integrin clustering mechanisms explored with a soluble αIIbβ3 ectodomain construct

The purpose of this study was to test the hypothesis that residues critical for ligand- and temperature-induced clustering of integrin alphaIIbbeta3 are present on its extracellular domain. Sucrose density gradient sedimentation was used to examine the effects of ligand-mimetic peptides and physiological temperature on the oligomeric state of a soluble recombinant ectodomain variant of the alphaIIbbeta3 integrin, alphaIIbDelta962beta3Delta692, and its full-length counterpart. Both the ectodomain construct, isolated from High Five insect cell culture supernatants, and alphaIIbbeta3, isolated from human blood platelets, exhibited similar weight-average sedimentation coefficients at 23 degrees C, in the absence and presence of the ligand-mimetic peptide eptifibatide. These observations indicate that alphaIIbbeta3's ectodomain exhibits a similar extended conformation in both its free and ligand-bound states. Oligomerization was examined by incubation of both alphaIIbDelta962beta3Delta692 and full-length receptors at 37 degrees C, in the presence or absence of ligand-mimetic. Minimal oligomerization was observed with alphaIIbDelta962beta3Delta692. In contrast, full-length alphaIIbbeta3 exhibited substantial temperature-induced increases in its distribution of sedimenting species, indicative of thermal aggregation. These observations suggest that optimum oligomerization requires the participation of the integrin's transmembrane and cytoplasmic regions. In vivo, clustering of ligand-bound integrins may enhance signaling by increasing the local concentration of intracellular integrin-associated proteins.

Integrins: dynamic scaffolds for adhesion and signaling in platelets

The major platelet integrin, alphaIIbbeta3, is required for platelet interactions with proteins in plasma and the extracellular matrices (ECMs) that are essential for platelet adhesion and aggregation during hemo stasis and arterial thrombosis. Lig and binding to alphaIIbbeta3 is controlled by inside-out signals that modulate receptor conformation and clustering. In turn, ligand binding triggers outside-in signals through alphaIIbbeta3 that, when disrupted, can cause a bleeding diathesis. In the past 5 years there has been an explosion of knowledge about the structure and function ofalphaIIbbeta3 and the related integrin, alphaVbeta3. These developments are discussed here, and current models of bidirectional alphaIIbbeta3 signaling are presented as frameworks for future investigations. An understanding that alphaIIbbeta3 functions as a dynamic molecular scaffold for extracellular and intracellular proteins has translated into diagnostic and therapeutic insights relevant to hematology and cardiovascular medicine, and further advances can be anticipated.

High affinity ligand binding by integrins does not involve head separation

Conformational change in the integrin extracellular domain is required for high affinity ligand binding and is also involved in post-ligand binding cellular signaling. Although there is evidence to the contrary, electron microscopic studies showing that ligand binding triggers alpha- and beta-subunit dissociation in the integrin headpiece have gained popularity and support the hypothesis that head separation activates integrins. To test directly the head separation hypothesis, we enforced head association by introducing disulfide bonds across the interface between the alpha-subunit beta-propeller domain and the beta-subunit I-like domain. Basal and activation-dependent ligand binding by alpha(IIb)beta(3) and alpha(V)beta(3) was unaffected. The covalent linkage prevented dissociation of alpha(IIb)beta(3) into its subunits on EDTA-treated cells. Whereas EDTA dissociated wild type alpha(IIb)beta(3) on the cell surface, a ligand-mimetic Arg-Gly-Asp peptide did not, as judged by binding of complex-specific antibodies. Finally, a high affinity ligand-mimetic compound stabilized noncovalent association between alpha(IIb) and beta(3) headpiece fragments in the presence of SDS, indicating that ligand binding actually stabilized subunit association at the head, as opposed to the suggested subunit separation. The mechanisms of conformational regulation of integrin function should therefore be considered in the context of the associated alphabeta headpiece.

Activation of integrin alphaIIbbeta3 by modulation of transmembrane helix associations

Transmembrane helices of integrin alpha and beta subunits have been implicated in the regulation of integrin activity. Two mutations, glycine-708 to asparagine-708 (G708N)and methionine-701 to asparagine-701, in the transmembrane helix of the beta3 subunit enabled integrin alphaIIbbeta3 to constitutively bind soluble fibrinogen. Further characterization of the G708N mutant revealed that it induced alphaIIbbeta3 clustering and constitutive phosphorylation of focal adhesion kinase. This mutation also enhanced the tendency of the transmembrane helix to form homotrimers. These results suggest that homomeric associations involving transmembrane domains provide a driving force for integrin activation. They also suggest a structural basis for the coincidence of integrin activation and clustering.

Detection of integrin alpha IIbbeta 3 clustering in living cells

In platelets, bidirectional signaling across integrin alpha(IIb)beta(3) regulates fibrinogen binding, cytoskeletal reorganization, cell aggregation, and spreading. Because these responses may be influenced by the clustering of alpha(IIb)beta(3) heterodimers into larger oligomers, we established two independent methods to detect integrin clustering and evaluate factors that regulate this process. In the first, weakly complementing beta-galactosidase mutants were fused to the C terminus of individual alpha(IIb) subunits, and the chimeras were stably expressed with beta(3) in Chinese hamster ovary cells. Clustering of alpha(IIb)beta(3) should bring the mutants into proximity and reconstitute beta-galactosidase activity. In the second method, alpha(IIb) was fused to either a green fluorescent protein (GFP) or Renilla luciferase and transiently expressed with beta(3). Here, integrin clustering should stimulate bioluminescence resonance energy transfer between a cell-permeable luciferase substrate and GFP. These methods successfully detected integrin clustering induced by anti-alpha(IIb)beta(3) antibodies. Significantly, they also detected clustering upon soluble fibrinogen binding to alpha(IIb)beta(3). In contrast, no clustering was observed following direct activation of alpha(IIb)beta(3) by MnCl(2) or an anti-alpha(IIb)beta(3)-activating antibody Fab in the absence of fibrinogen. Intracellular events also influenced alpha(IIb)beta(3) clustering. For example, a cell-permeable, bivalent FK506-binding protein (FKBP) ligand stimulated clustering when added to cells expressing an alpha(IIb)(FKBP)(2) chimera complexed with beta(3). Furthermore, alpha(IIb)beta(3) clustering occurred in the presence of latrunculin A or cytochalasin D, inhibitors of actin polymerization. These effects were enhanced by fibrinogen, suggesting that actin-regulated clustering modulates alpha(IIb)beta(3) interaction with ligands. These studies in living cells establish that alpha(IIb)beta(3) clustering is modulated by fibrinogen and actin dynamics. More broadly, they should facilitate investigations of the mechanisms and consequences of integrin clustering.

Stabilizing the open conformation of the integrin headpiece with a glycan wedge increases affinity for ligand

The affinity of the extracellular domain of integrins for ligand is regulated by conformational changes signaled from the cytoplasm. Alternative types of conformational movement in the ligand-binding headpiece have been proposed. In one study, electron micrograph image averages of the headpiece of integrin aV beta 3 show two different conformations. The open conformation of the headpiece is present when a ligand mimetic peptide is bound and differs from the closed conformation in the presence of an obtuse angle between the beta 3 subunit hybrid and I-like domains. We tested the hypothesis that opening of the hybrid-I-like domain interface increases ligand-binding affinity by mutationally introducing an N-glycosylation site into it. Both beta 3 and beta1 integrin glycan wedge mutants exhibit constitutively high affinity for physiological ligands. The data uniquely support one model of integrin activation and suggest that movement at the interface with the hybrid domain pulls down the C-terminal helix of the I-like domain and activates its metal ion-dependent adhesion site, analogously to activation of the integrin I domain.