A novel adaptation of the integrin PSI domain revealed from its crystal structure

Rationally designed integrin beta3 mutants stabilized in the high affinity conformation

Integrins are important cell surface receptors that transmit bidirectional signals across the membrane. It has been shown that a conformational change of the integrin beta-subunit headpiece (i.e. the beta I domain and the hybrid domain) plays a critical role in regulating integrin ligand binding affinity and function. Previous studies have used coarse methods (a glycan wedge, mutations in transmembrane contacts) to force the beta-subunit into either the open or closed conformation. Here, we demonstrate a detailed understanding of this conformational change by applying computational design techniques to select five amino acid side chains that play an important role in the energetic balance between the open and closed conformations of alphaIIbbeta3. Eight single-point mutants were designed at these sites, of which five bound ligands much better than wild type. Further, these mutants were found to be in a more extended conformation than wild type, suggesting that the conformational change at the ligand binding headpiece was propagated to the legs of the integrin. This detailed understanding of the conformational change will assist in the development of allosteric drugs that either stabilize or destabilize specific integrin conformations without occluding the ligand-binding site.

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.

The Role of Integrins in Cancer and the Development of Anti-Integrin Therapeutic Agents for Cancer Therapy

Integrins have been reported to mediate cell survival, proliferation, differentiation, and migration programs. For this reason, the past few years have seen an increased interest in the implications of integrin receptors in cancer biology and tumor cell aggression. This review considers the potential role of integrins in cancer and also addresses why integrins are present attractive targets for drug design. It discusses of the several properties of the integrin-based chemotherapeutic agents currently under consideration clinically and provides an insight into cancer drug development using integrin as a target.

Multivalent presentation of antihantavirus peptides on nanoparticles enhances infection blockade

Viral entry into susceptible host cells typically results from multivalent interactions between viral surface proteins and host entry receptors. In the case of Sin Nombre virus (SNV), a New World hantavirus that causes hantavirus cardiopulmonary syndrome, infection involves the interaction between viral membrane surface glycoproteins and the human integrin alpha(v)beta(3). Currently, there are no therapeutic agents available which specifically target SNV. To address this problem, we used phage display selection of cyclic nonapeptides to identify peptides that bound SNV and specifically prevented SNV infection in vitro. We synthesized cyclic nonapeptides based on peptide sequences of phage demonstrating the strongest inhibition of infection, and in all cases, the isolated peptides were less effective at blocking infection (9.0% to 27.6% inhibition) than were the same peptides presented by phage (74.0% to 82.6% inhibition). Since peptides presented by the phage were pentavalent, we determined whether the identified peptides would show greater inhibition if presented in a multivalent format. We used carboxyl linkages to conjugate selected cyclic peptides to multivalent nanoparticles and tested infection inhibition. Two of the peptides, CLVRNLAWC and CQATTARNC, showed inhibition that was improved over that of the free format when presented on nanoparticles at a 4:1 nanoparticle-to-virus ratio (9.0% to 32.5% and 27.6% to 37.6%, respectively), with CQATTARNC inhibition surpassing 50% when nanoparticles were used at a 20:1 ratio versus virus. These data illustrate that multivalent inhibitors may disrupt polyvalent protein-protein interactions, such as those utilized for viral infection of host cells, and may represent a useful therapeutic approach.

Role of the beta-subunit arginine/lysine finger in integrin heterodimer formation and function

Formation of the integrin alphabeta heterodimer is essential for cell surface expression and function. At the core of the alphabeta interface is a conserved Arg/Lys "finger" from the beta-subunit that inserts into a cup-like "cage" formed of two layers of aromatic residues in the alpha-subunit. We evaluated the role of this residue in heterodimer formation in an alphaA-lacking and an alphaA-containing integrin alphaVbeta3 and alphaMbeta2 (CD11b/CD18), respectively. Arg261 of beta3 was mutated to Ala or Glu; the corresponding Lys252 of beta2 was mutated to Ala, Arg, Glu, Asp, or Phe; and the effects on heterodimer formation in each integrin examined by ELISA and immunoprecipitation in HEK 293 cells cotransfected with plasmids encoding the alpha- and beta-subunits. The Arg261Glu (but not Arg261Ala) substitution significantly impaired cell surface expression and heterodimer formation of alphaVbeta3. Although Lys252Arg, and to a lesser extent Lys252Ala, were well tolerated, each of the remaining substitutions markedly reduced cell surface expression and heterodimer formation of CD11b/CD18. Lys252Arg and Lys252Ala integrin heterodimers displayed a significant increase in binding to the physiologic ligand iC3b. These data demonstrate an important role of the Arg/Lys finger in formation of a stable integrin heterodimer, and suggest that subtle changes at this residue affect the activation state of the integrin.

Immunologic and structural analysis of eight novel domain-deletion beta3 integrin peptides designed for detection of HPA-1 antibodies

BACKGROUND

The single-nucleotide polymorphism (SNP) rs5918 in the ITGB3 gene defines the human platelet antigen-1 (HPA-1) system encoding a Leu (HPA-1a) or Pro (HPA-1b) at position 33. HPA-1 antibodies are clinically the most relevant in the Caucasoid population, but detection currently requires alpha(IIb)beta3 integrin from the platelets of HPA-genotyped donors.

OBJECTIVES

We set out to define the beta3 integrin domains required for HPA-1a antibody binding and produce recombinant soluble beta3 peptides for HPA-1 antibody detection.

METHODS

We designed two sets (1a and 1b) of four soluble beta3 domain-deletion peptides (deltaSDL, deltabetaA, PSIHybrid, PSI), informed by crystallography studies and computer modeling. The footprints of three human HPA-1a-specific phage antibodies were defined by analyzing binding patterns to the beta3 peptides and canine platelets, and models of antibody-antigen interfaces were derived. Specificity and sensitivity for HPA-1a detection were assessed using sera from 140 cases of fetomaternal alloimmune thrombocytopenia (FMAIT).

RESULTS

Fusion of recombinant proteins to calmodulin resulted in high-level expression in Drosophila S2 cells of all eight beta3 peptides. Testing of FMAIT samples indicated that deltabetaA-Leu33 is the superior peptide for HPA-1a antibody detection, with 96% sensitivity and 95% specificity. The existence of type I and II categories of HPA-1a antibodies was confirmed by the study of HPA-1a phage antibody footprints and the reactivity pattern of clinical samples with the four beta3-Leu33 peptides, but there was no correlation between antibody category and clinical severity of FMAIT.

CONCLUSIONS

Soluble recombinant beta3 peptides can be used for detection of clinical HPA-1a antibodies.

Increased activity of phosphatase PP2A in the presence of the PlA2 polymorphism of alphaIIbbeta3

Polymorphisms in alphaIIbbeta3 are important genetic factors that alter platelet biology and have been associated with susceptibility to thromboembolic disorders. To define the molecular mechanisms that lead to variance in thrombotic diathesis dictated by the beta3 polymorphism, we examined regulation of intracellular signaling by alphaIIbbeta3, and studied the effects of a common beta subunit PlA2 polymorphism. We found that PP2A regulates alphaIIbbeta3 control of the ERK signaling in a polymorphism specific fashion. In CHO cells, exogenous expression of alphaIIbbeta3 reduced ATP-stimulated ERK phosphorylation and more so for PlA2 than PlA1. Interestingly, reduced level of ERK phosphorylation correlated with an increase in PP2A activity, with higher activity associated with PlA2 than PlA1. We tested the effect of PP2A on alphaIIbbeta3-dependent adhesion, and found that PP2A overexpression increased cell adhesion, while phosphatase inhibitors decreased cell adhesion. We propose that PlA2 alters cell signaling at least in part by increasing beta3-associated PP2A activity.

Structure and mechanics of integrin-based cell adhesion

Integrins are alpha/beta heterodimeric adhesion glycoprotein receptors that regulate a wide variety of dynamic cellular processes such as cell migration, phagocytosis, and growth and development. X-ray crystallography of the integrin ectodomain revealed its modular architecture and defined its metal-dependent interaction with extracellular ligands. This interaction is regulated from inside the cell (inside-out activation), through the short cytoplasmic alpha and beta integrin tails, which also mediate biochemical and mechanical signals transmitted to the cytoskeleton by the ligand-occupied integrins, effecting major changes in cell shape, behavior, and fate. Recent advances in the structural elucidation of integrins and integrin-binding cytoskeleton proteins are the subjects of this review.

A novel structural fold in polysaccharide lyases: Bacillus subtilis family 11 rhamnogalacturonan lyase YesW with an eight-bladed beta-propeller

Rhamnogalacturonan (RG) lyase produced by plant pathogenic and saprophytic microbes plays an important role in degrading plant cell walls. An extracellular RG lyase YesW from saprophytic Bacillus subtilis is a member of polysaccharide lyase family 11 and cleaves glycoside bonds in polygalacturonan as well as RG type-I through a beta-elimination reaction. Crystal structures of YesW and its complex with galacturonan disaccharide, a reaction product analogue, were determined at 1.4 and 2.5 A resolutions with final R-factors of 16.4% and 16.6%, respectively. The enzyme is composed of an eight-bladed beta-propeller with a deep cleft in the center as a basic scaffold, and its structural fold has not been seen in polysaccharide lyases analyzed thus far. Structural analysis of the disaccharide-bound YesW and a site-directed mutagenesis study suggested that Arg-452 and Lys-535 stabilize the carboxyl group of the acidic polysaccharide molecule and Tyr-595 makes a stack interaction with the sugar pyranose ring. In addition to amino acid residues binding to the disaccharide, one calcium ion, which is coordinated by Asp-401, Glu-422, His-363, and His-399, may mediate the enzyme activity. This is, to our knowledge, the first report of a new structural category with a beta-propeller fold in polysaccharide lyases and provides structural insights into substrate binding by RG lyase.

Rapid detection of HPA-1 alloantibodies by platelet antigens immobilized onto microbeads

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT) is one of the most common bleeding disorders in neonates. It occurs when alloantibodies from an immunized mother react with paternally inherited alloantigens, mostly human platelet antigen 1a (HPA-1a), on the fetal platelets (PLTs). Currently, monoclonal antibody-immobilized PLT antigen (MAIPA) assay represents the standard technique for the serologic diagnosis of NAIT. MAIPA is time-consuming, however, and limited by the availability of monoclonal antibodies (MoAbs). Here, a gel antigen-specific assay (GASA) was developed, which allows rapid detection of HPA-1 alloantibodies without the use of MoAbs.

STUDY DESIGN AND METHODS

Glycoprotein (GP) IIb/IIIa was purified by affinity chromatography from outdated PLT concentrates derived from HPA-1aa or HPA-1bb donors. Purified GPs were biotinylated, immobilized onto streptavidin beads, and used for the analysis of HPA-1a alloantibodies by a microtyping system. HPA-1a serum samples derived from mothers with NAIT (n = 36) and from posttransfusion purpura patients (n = 2) as well as HPA-1b (n = 4), HPA-5b (n = 2), HPA-3a (n = 4), and HLA Class I (n = 2) alloantiserum samples from multitransfused patients were investigated in GASA and MAIPA assays.

RESULTS

GASA was able to detect all HPA-1a and -1b alloantibodies recognized by MAIPA. Cross-reactivity with other PLT-reactive alloantibodies was not observed. Interestingly, 3 of 36 serum samples, which showed only moderate reactivity in MAIPA, reacted strongly in GASA.

CONCLUSION

GASA has proved to be a rapid method for the detection of HPA-1a alloantibodies and maybe useful for PLT antibody screening, especially in initial assessment of suspected NAIT cases.

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.

Characterization and NMR solution structure of a novel cyclic pentapeptide inhibitor of pathogenic hantaviruses

Hantavirus-induced diseases such as hantavirus cardiopulmonary syndrome and hemorrhagic fever with renal syndrome are a global health concern. Hantavirus cardiopulmonary syndrome caused by Sin Nombre virus lacks specific therapy and its high mortality makes Sin Nombre virus a potential bioweapon agent. Sin Nombre virus entry into susceptible cells requires expression of alpha(v)beta(3) integrin. We recently reported the sequence of a cyclic nonapeptide that inhibited Sin Nombre virus entry into Vero E6 cells at a level comparable to ReoPro, a Fab fragment of the anti-beta(3) antibody c7E3. Here, we refine the parental peptide, cyclo-[CPFVKTQLC], using alanine scanning and amino acid deletions, by optimizing for viral inhibition. The IC(50) of the resulting peptide, cyclo-[CPFVC], was 267 microM compared with 263 microM for the parental peptide. The solution structure of cyclo-[CPFVC] was determined by two-dimensional nuclear magnetic resonance spectroscopy, revealing the Phe ring in an extended conformation stacked against the Pro ring and containing a beta-turn encompassing Val-4 through Cys-1. As an initial step in identifying interactions between cyclo-[CPFVC] and its target cellular receptor, the refined peptide structure was docked into the ReoPro binding site of integrin beta(3). This structure will provide the basis for designing more potent peptidomimetic therapeutics to prevent Sin Nombre virus entry and treat hantavirus cardiopulmonary syndrome.

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.

Mapping early conformational changes in alphaIIb and beta3 during biogenesis reveals a potential mechanism for alphaIIbbeta3 adopting its bent conformation

Current evidence supports a model in which the low-affinity state of the platelet integrin alphaIIbbeta3 results from alphaIIbbeta3 adopting a bent conformation. To assess alphaIIbbeta3 biogenesis and how alphaIIbbeta3 initially adopts the bent conformation, we mapped the conformational states occupied by alphaIIb and beta3 during biogenesis using conformation-specific monoclonal antibodies (mAbs). We found that alphaIIbbeta3 complex formation was not limited by the availability of either free pro-alphaIIb or free beta3, suggesting that other molecules, perhaps chaperones, control complex formation. Five beta3-specific, ligand-induced binding site (LIBS) mAbs reacted with much or all free beta3 but not with beta3 when in complex with mature alphaIIb, suggesting that beta3 adopts its mature conformation only after complex formation. Conversely, 2 alphaIIb-specific LIBS mAbs directed against the alphaIIb Calf-2 region adjacent to the membrane reacted with only minor fractions of free pro-alphaIIb, raising the possibility that pro-alphaIIb adopts a bent conformation early in biogenesis. Our data suggest a working model in which pro-alphaIIb adopts a bent conformation soon after synthesis, and then beta3 assumes its bent conformation by virtue of its interaction with the bent pro-alphaIIb.

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.

The beta-tail domain (betaTD) regulates physiologic ligand binding to integrin CD11b/CD18

Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (betaTD) and the ligand-binding betaA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alphaMbeta2) with its physiologic ligand iC3b. We replaced the betaTD CD loop residues D658GMD of the CD18 (beta2) subunit with the equivalent D672SSG of the beta3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca(2+) plus Mg(2+) (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the betaTD-modified integrins bound in Ca(2+)Mg(2+) to the betaA-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal betaTD as an allosteric modulator of integrin activation.

Activation of leukocyte beta2 integrins by conversion from bent to extended conformations

We used negative stain electron microscopy (EM) to examine the conformational changes in the ectodomains required for activation of the leukocyte integrins alpha(X)beta(2) and alpha(L)beta(2). They transitioned between a bent conformation and two extended conformations in which the headpiece was in either a closed or an open state. Extended integrins exhibited marked flexibility at the alpha subunit genu and between integrin epidermal growth factor-like (I-EGF) domains 1 and 2. A clasp to mimic juxtamembrane association between the integrin alpha and beta subunits stabilized the bent conformation strongly for alpha(X)beta(2) and less so for alpha(L)beta(2). A small molecule allosteric antagonist induced the extended, open headpiece conformation. A Fab known to activate beta(2) integrins on leukocytes induced extension, and a Fab reporter of activation bound only after extension had been induced. The results establish an intimate relationship between extension of beta(2) integrins and their activation in immune responses and leukocyte trafficking.

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.

The effect of the single substitution of arginine within the RGD tripeptide motif of a modified neurotoxin dendroaspin on its activity of platelet aggregation and cell adhesion

The Arg-Gly-Asp (RGD) tripeptide unit is a cell-cell and cell-extracellular matrix recognition sequence of some integrins that is found within several extracellular matrix glycoproteins and dendroaspin, a disintegrin-like venom protein isolated from the snake venom of the Dendroaspis jamsonii. In the present study, the RGD motif in dendroaspin was substituted by Lys-Gly-Asp (KGD), His-Gly-Asp (HGD), Gln-Gly-Asp (QGD) and Ala-Gly-Asp (AGD) denoted as KGD-den, HGD-den, QGD-den and AGD-den, respectively. Each of the mutants exhibited activity as inhibitor of ADP-induced platelet aggregation with IC50 values of 0.26, 2.5, 6, and 17 microM for KGD-den, HGD-den, QGD-den, and AGD-den, respectively, as compared with RGD-den (IC50 = 0.18 microM). Interestingly, HGD-den was approx. two-fold more potent and a more selective inhibitor than either the KGD-den or QGD-den counterpart at blocking A375-SM human melanoma cell adhesion to fibrinogen (beta3-mediated). KGD-den, HGD-den, and QGD-den were preferentially antagonists of A375-SM human melanoma cell adhesion to fibrinogen rather than to fibronectin (alpha5beta1-, beta3-mediated). Both HGD-den and KGD-den were equipotent as inhibitors of human erythroleukaemia (HEL) cell adhesion to fibrinogen (IC50 = 0.15 microM) and also preferential inhibitors of HEL cell adhesion to fibrinogen (beta3 and beta1-mediated) rather than to fibronectin. These findings show that the presence of the arginine within the RGD motif of dendroaspin is not obligatory and substitution of this residue can modulate inhibitory potency and integrin binding selectivity.

A naturally occurring Leu33Val mutation in beta3-integrin impairs the HPA-1a epitope: the third allele of HPA-1

BACKGROUND

Single-amino-acid substitution Leu33Pro in the beta3-integrin is responsible for the formation of the human platelet antigen (HPA)-1. Alloimmunization against HPA-1a (beta3-Leu33) is the most frequent cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura.

STUDY DESIGN AND METHODS

While HPA-1 genotyping a large cohort of patients with thromboembolic disease with a thermal cycler (LightCycler), one patient was identified with a unique HPA-1a melting curve.

RESULTS

Sequence analysis revealed a C-to-G transversion at nucleotide 175 in the beta3-integrin (ITGB3) gene that alters the Leu33 codon to Val33. Further genotyping of healthy blood donors (n = 2950) identified one nonrelated Pro33Val33-positive individual. To examine whether the presence of Val33 affected the binding pattern of HPA-1 alloantibodies, transfectants were generated expressing recombinant beta3-Leu33 or beta3-Val33. Interestingly, differences in the reactivity of anti-HPA-1a were observed, with some HPA-1a alloantibodies showing diminished reactivity with beta3-Val33 compared to beta3-Leu33 and others reacting equally with both types. Similar findings were observed with recombinant human HPA-1a antibodies, with one of the three not binding to beta3-Val33.

CONCLUSIONS

Our results demonstrate that the naturally occurring Leu33Val mutation in the beta3-integrin can disrupt some HPA-1a epitopes. These findings provide evidence for a heterogeneous humoral response against HPA-1a that may have potential clinical implications for alloimmune thrombocytopenia disorders.

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.

Targeting integrin structure and function in disease

Initially linked to the pathogenesis of inflammatory and hematologic diseases, integrins have become validated drug targets with the approval of five drugs. Moreover, there are several promising drug candidates in preclinical and clinical stages of development for multiple clinical indications. Integrins are attractive drug targets as their antagonism can block several steps in disease progression or maintenance. Integrin inhibitors can block the proliferation, migration, or tissue localization of inflammatory, angiogenic, and tumor cells, as well as signaling and gene expression contributing to disease. There has been a rapid increase in the elucidation of integrin structure, their allosteric mechanisms of bidirectional signaling, and the structure of complexes with drugs. This information brings greater focus to how integrins support various cellular functions and how they have been and may be targeted to develop novel drugs. Here we review conformational switches, including an internal ligand, which allosterically regulate the transition from low- to high-affinity ligand binding. We address some of the successes, disappointments, and challenges in targeting competitive or allosteric sites to develop therapeutics. We also discuss new opportunities, including a structure-based approach to discover novel drugs to treat inflammatory and other diseases. This approach targets structural relatives of the von Willebrand factor A-domain present in integrins and many functionally diverse proteins.

Amino acid changes in Drosophila alphaPS2betaPS integrins that affect ligand affinity

We developed a ligand-mimetic antibody Fab fragment specific for Drosophila alphaPS2betaPS integrins to probe the ligand binding affinities of these invertebrate receptors. TWOW-1 was constructed by inserting a fragment of the extracellular matrix protein Tiggrin into the H-CDR3 of the alphavbeta3 ligand-mimetic antibody WOW-1. The specificity of alphaPS2betaPS binding to TWOW-1 was demonstrated by numerous tests used for other integrin-ligand interactions. Binding was decreased in the presence of EDTA or RGD peptides and by mutation of the TWOW-1 RGD sequence or the betaPS metal ion-dependent adhesion site (MIDAS) motif. TWOW-1 binding was increased by mutations in the alphaPS2 membrane-proximal cytoplasmic GFFNR sequence or by exposure to Mn2+. Although Mn2+ is sometimes assumed to promote maximal integrin activity, TWOW-1 binding in Mn2+ could be increased further by the alphaPS2 GFFNR --> GFANA mutation. A mutation in the betaPS I domain (betaPS-b58; V409D) greatly increased ligand binding affinity, explaining the increased cell spreading mediated by alphaPS2betaPS-b58. Further mutagenesis of this residue suggested that Val-409 normally stabilizes the closed head conformation. Mutations that potentially reduce interaction of the integrin beta subunit plexin-semaphorin-integrin (PSI) and stalk domains have been shown to have activating properties. We found that complete deletion of the betaPS PSI domain enhanced TWOW-1 binding. Moreover the PSI domain is dispensable for at least some other integrin functions because betaPS-DeltaPSI displayed an enhanced ability to mediate cell spreading. These studies establish a means to evaluate mechanisms and consequences of integrin affinity modulation in a tractable model genetic system.

The crystal structure of the plexin-semaphorin-integrin domain/hybrid domain/I-EGF1 segment from the human integrin beta2 subunit at 1.8-A resolution

Integrins are modular (alphabeta) heterodimeric proteins that mediate cell adhesion and convey signals across the plasma membrane. Interdomain motions play a key role in signal transduction by propagating structural changes through the molecule, thus controlling the activation state and adhesive properties of the integrin. We expressed a soluble fragment of the human integrin beta2 subunit comprising the plexin-semaphorin-integrin domain (PSI)/hybrid domain/I-EGF1 fragment and present its crystal structure at 1.8-A resolution. The structure reveals an elongated molecule with a rigid architecture stabilized by nine disulfide bridges. The PSI domain is located centrally and participates in the formation of extended interfaces with the hybrid domain and I-EGF1 domains, respectively. The hybrid domain/PSI interface involves the burial of an Arg residue, and contacts between PSI and I-EGF1 are mainly mediated by well conserved Arg and Trp residues. Conservation of key interacting residues across the various integrin beta subunits sequences suggests that our structure represents a good model for the entire integrin family. Superposition with the integrin beta3 receptor in its bent conformation suggests that an articulation point is present at the linkage between its I-EGF1 and I-EGF2 modules and underlines the importance of this region for the control of integrin-mediated cell adhesion.

Three-dimensional EM structure of the ectodomain of integrin {alpha}V{beta}3 in a complex with fibronectin

Integrins are αβ heterodimeric cell surface receptors that mediate transmembrane signaling by binding extracellular and cytoplasmic ligands. The ectodomain of integrin αVβ3 crystallizes in a bent, genuflexed conformation considered to be inactive (unable to bind physiological ligands in solution) unless it is fully extended by activating stimuli. We generated a stable, soluble complex of the Mn²⁺-bound αVβ3 ectodomain with a fragment of fibronectin (FN) containing type III domains 7 to 10 and the EDB domain (FN7-EDB-10). Transmission electron microscopy and single particle image analysis were used to determine the three-dimensional structure of this complex. Most αVβ3 particles, whether unliganded or FN-bound, displayed compact, triangular shapes. A difference map comparing ligand-free and FN-bound αVβ3 revealed density that could accommodate the RGD-containing FN10 in proximity to the ligand-binding site of β3, with FN9 just adjacent to the synergy site binding region of αV. We conclude that the ectodomain of αVβ3 manifests a bent conformation that is capable of stably binding a physiological ligand in solution.

Evidence that monoclonal antibodies directed against the integrin beta subunit plexin/semaphorin/integrin domain stimulate function by inducing receptor extension

The overall structure of integrins is that of a ligand-binding head connected to two long legs. The legs can exhibit a pronounced bend at the "knees," and it has been proposed that the legs undergo a dramatic straightening when integrins transit from a low affinity to a high affinity state. The knee region contains domains from both alpha and beta subunits, including the N-terminal plexin/semaphorin/integrin (PSI) domain of the beta subunit. The role played by the knee domains in the regulation of integrin-ligand binding is uncertain. Here we show that: (i) monoclonal antibodies (mAbs) N29 and 8E3 have epitopes in the beta(1) subunit PSI domain and stimulate ligand binding to alpha(5)beta(1); (ii) N29 and 8E3 cause long range conformational changes that alter the ligand binding activity of the head region; (iii) the stimulatory action of these mAbs is dependent on the calf-1 domain, which forms part of the alpha subunit knee; and (iv) the epitopes of 8E3 and N29 map close to the extreme N terminus of the PSI and are likely to lie on the side of this domain that faces the alpha subunit. Taken together, our data suggest that the binding of these mAbs results in a levering apart of the PSI and calf-1 domains, and thereby causes the alpha and beta subunit knees to separate. Several major inferences can be drawn from our findings. First, the PSI domain appears to form part of an interface with the alpha subunit that normally restrains the integrin in a bent state. Second, the PSI domain is important for the transduction of conformational changes from the knee to head. Third, unbending is likely to provide a general mechanism for control of integrin-ligand recognition.