Evidence for a differential functional regulation of the two beta(3)-integrins alpha(V)beta(3) and alpha(IIb)beta(3)

Conformational response of αIIbβ3 and αVβ3 integrins to force

As major adhesion receptors, integrins transmit biochemical and mechanical signals across the plasma membrane. These functions are regulated by transitions between bent and extended conformations and modulated by force. To understand how force on integrins mediates cellular mechanosensing, we compared two highly homologous integrins, αIIbβ3 and αVβ3. These integrins, expressed in circulating platelets vs. solid tissues, respectively, share the β3 subunit, bind similar ligands and have similar bent and extended conformations. Here, we report that in cells expressing equivalent levels of each integrin, αIIbβ3 mediates spreading on softer substrates than αVβ3. These effects correlate with differences in structural dynamics of the two integrins under force. All-atom simulations show that αIIbβ3 is more flexible than αVβ3 due to correlated residue motions within the α subunit domains. Single molecule measurements confirm that αIIbβ3 extends faster than αVβ3. These results reveal a fundamental relationship between protein function and structural dynamics in cell mechanosensing.

The α1 integrin cytoplasmic tail interacts with phosphoinositides and interferes with Akt activation

Integrin α1β1 is an adhesion receptor that binds to collagen and laminin. It regulates cell adhesion, cytoskeletal organization, and migration. The cytoplasmic tail of the α1 subunit consists of 15 amino acids and contains six positively charged lysine residues. In this study, we present evidence that the α1 integrin cytoplasmic tail (α1CT) directly associates with phosphoinositides, preferentially with phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3). Since the association was disrupted by calcium, magnesium and phosphate ions, this interaction appears to be in ionic nature. Here, the peptide-lipid interaction was driven by the conserved KIGFFKR motif. The exchange of both two potential phospholipid-binding lysines for glycines in the KIGFFKR motif increased α1β1 integrin-specific adhesion and F-actin cytoskeleton formation compared to cells expressing the unmodified α1 subunit, whereas only mutation of the second lysine at position 1171 increased levels of constitutively active α1β1 integrins on the cell surface. In addition, enhanced focal adhesion formation and increased phosphorylation of focal adhesion kinase, but decreased phosphorylation of AKT was observed in these cells. We conclude that the KIGFFKR motif, and in particular lysine1171 is involved in the dynamic regulation of α1β1 integrin activity and that the interaction of α1CT with phosphoinositides may contribute to this process.

Identification of Equine Lactadherin-derived Peptides That Inhibit Rotavirus Infection via Integrin Receptor Competition

Human rotavirus is the leading cause of severe gastroenteritis in infants and children under the age of 5 years in both developed and developing countries. Human lactadherin, a milk fat globule membrane glycoprotein, inhibits human rotavirus infection in vitro, whereas bovine lactadherin is not active. Moreover, it protects breastfed infants against symptomatic rotavirus infections. To explore the potential antiviral activity of lactadherin sourced by equines, we undertook a proteomic analysis of milk fat globule membrane proteins from donkey milk and elucidated its amino acid sequence. Alignment of the human, bovine, and donkey lactadherin sequences revealed the presence of an Asp-Gly-Glu (DGE) α2β1 integrin-binding motif in the N-terminal domain of donkey sequence only. Because integrin α2β1 plays a critical role during early steps of rotavirus host cell adhesion, we tested a minilibrary of donkey lactadherin-derived peptides containing DGE sequence for anti-rotavirus activity. A 20-amino acid peptide containing both DGE and RGD motifs (named pDGE-RGD) showed the greatest activity, and its mechanism of antiviral action was characterized; pDGE-RGD binds to integrin α2β1 by means of the DGE motif and inhibits rotavirus attachment to the cell surface. These findings suggest the potential anti-rotavirus activity of equine lactadherin and support the feasibility of developing an anti-rotavirus peptide that acts by hindering virus-receptor binding.

β2 integrin mediates hantavirus-induced release of neutrophil extracellular traps

β2 Integrin–mediated systemic release of neutrophil extracellular traps is a novel mechanism of immunopathology associated with hantavirus infection.

Rodent-borne hantaviruses are emerging human pathogens that cause severe human disease. The underlying mechanisms are not well understood, as hantaviruses replicate in endothelial and epithelial cells without causing any cytopathic effect. We demonstrate that hantaviruses strongly stimulated neutrophils to release neutrophil extracellular traps (NETs). Hantavirus infection induced high systemic levels of circulating NETs in patients and this systemic NET overflow was accompanied by production of autoantibodies to nuclear antigens. Analysis of the responsible mechanism using neutrophils from β2 null mice identified β2 integrin receptors as a master switch for NET induction. Further experiments suggested that β2 integrin receptors such as complement receptor 3 (CR3) and 4 (CR4) may act as novel hantavirus entry receptors. Using adenoviruses, we confirmed that viral interaction with β2 integrin induced strong NET formation. Collectively, β2 integrin–mediated systemic NET overflow is a novel viral mechanism of immunopathology that may be responsible for characteristic aspects of hantavirus-associated disease such as kidney and lung damage.

Integrin α1 has a long helix, extending from the transmembrane region to the cytoplasmic tail in detergent micelles

Integrin proteins are very important adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. They play essential roles in cell signaling and the regulation of cellular shape, motility, and the cell cycle. Here, the transmembrane and cytoplasmic (TMC) domains of integrin α1 and β1 were over-expressed and purified in detergent micelles. The structure and backbone relaxations of α1-TMC in LDAO micelles were determined and analyzed using solution NMR. A long helix, extending from the transmembrane region to the cytoplasmic tail, was observed in α1-TMC. Structural comparisons of α1-TMC with reported αIIb-TMC domains indicated different conformations in the transmembrane regions and cytoplasmic tails. An NMR titration experiment indicated weak interactions between α1-TMC and β1-TMC through several α1-TMC residues located at its N-terminal juxta-transmembrane region and C-terminal extended helix region.

Dobrava-Belgrade hantavirus from Germany shows receptor usage and innate immunity induction consistent with the pathogenicity of the virus in humans

Background

Dobrava-Belgrade virus (DOBV) is a European hantavirus causing hemorrhagic fever with renal syndrome (HFRS) in humans with fatality rates of up to 12%. DOBV-associated clinical cases typically occur also in the northern part of Germany where the virus is carried by the striped field mouse (Apodemus agrarius). However, the causative agent responsible for human illness has not been previously isolated.

Methodology/Principal Findings

Here we report on characterization of a novel cell culture isolate from Germany obtained from a lung tissue of “spillover” infected yellow necked mouse (A. flavicollis) trapped near the city of Greifswald. Phylogenetic analyses demonstrated close clustering of the new strain, designated Greifswald/Aa (GRW/Aa) with the nucleotide sequence obtained from a northern German HFRS patient. The virus was effectively blocked by specific antibodies directed against β3 integrins and Decay Accelerating Factor (DAF) indicating that the virus uses same receptors as the highly pathogenic Hantaan virus (HTNV). In addition, activation of selected innate immunity markers as interferon β and λ and antiviral protein MxA after viral infection of A549 cells was investigated and showed that the virus modulates the first-line antiviral response in a similar way as HTNV.

Conclusions/Significance

In summary, our study reveals novel data on DOBV receptor usage and innate immunity induction in relationship to virus pathogenicity and underlines the potency of German DOBV strains to act as human pathogen.

Sangassou virus, the first hantavirus isolate from Africa, displays genetic and functional properties distinct from those of other murinae-associated hantaviruses

We have discovered the first indigenous African hantavirus, Sangassou virus (SANGV). The virus was isolated from an African wood mouse (Hylomyscus simus), trapped in a forest habitat in Guinea, West Africa. Here, we report on the characterization of the genetic and functional properties of the virus. The complete genome of SANGV was determined and showed typical hantavirus organization. The small (S), medium (M), and large (L) genome segments containing genes encoding nucleocapsid protein, two envelope glycoproteins, and viral polymerase were found to be 1,746, 3,650, and 6,531 nucleotides long, respectively. The exact 5' and 3' termini for all three segments of the SANGV genome were determined and were predicted to form the panhandle structures typical of bunyaviruses. Phylogenetic analyses of all three segment sequences confirmed SANGV as a Murinae-associated hantavirus most closely related to the European Dobrava-Belgrade virus. We showed, however, that SANGV uses β(1) integrin rather than β(3) integrin and decay-accelerating factor (DAF)/CD55 as an entry receptor. In addition, we demonstrated a strong induction of type III lambda interferon (IFN-λ) expression in type I IFN-deficient Vero E6 cells by SANGV. These properties are unique within Murinae-associated hantaviruses and make the virus useful in comparative studies focusing on hantavirus pathogenesis.

Glycoproteins D of equine herpesvirus type 1 (EHV-1) and EHV-4 determine cellular tropism independently of integrins

Equine herpesvirus type 1 (EHV-1) and EHV-4 are genetically and antigenically very similar, but their pathogenic potentials are strikingly different. The differences in pathogenicity between both viruses seem to be reflected in cellular host range: EHV-1 can readily be propagated in many cell types of multiple species, while EHV-4 entry and replication appear to be restricted mainly to equine cells. The clear difference in cellular tropism may well be associated with differences in the gene products involved in virus entry and/or spread from cell to cell. Here we show that (i) most of the EHV-1 permissive cell lines became resistant to EHV-1 expressing EHV-4 glycoprotein D (gD4) and the opposite was observed for EHV-4 harboring EHV-1 gD (gD1). (ii) The absence of integrins did not inhibit entry into and replication of EHV-1 in CHO-K1 or peripheral blood mononuclear cells (PBMC). Furthermore, integrin-negative K562 cells did not acquire the ability to bind to gD1 when αVβ3 integrin was overexpressed. (iii) PBMC could be infected with similar efficiencies by both EHV-1 and EHV-4 in vitro. (iv) In contrast to results for equine fibroblasts and cells of endothelial or epithelial origin, we were unable to block entry of EHV-1 or EHV-4 into PBMC with antibodies directed against major histocompatibility complex class I (MHC-I), a result that indicates that these viruses utilize a different receptor(s) to infect PBMC. Cumulatively, we provide evidence that efficient EHV-1 and EHV-4 entry is dependent mainly on gD, which can bind to multiple cell surface receptors, and that gD has a defining role with respect to cellular host range of EHV-1 and EHV-4.

Successful in vitro expansion and differentiation of cord blood derived CD34+ cells into early endothelial progenitor cells reveals highly differential gene expression

Endothelial progenitor cells (EPCs) can be purified from peripheral blood, bone marrow or cord blood and are typically defined by a limited number of cell surface markers and a few functional tests. A detailed in vitro characterization is often restricted by the low cell numbers of circulating EPCs. Therefore in vitro culturing and expansion methods are applied, which allow at least distinguishing two different types of EPCs, early and late EPCs. Herein, we describe an in vitro culture technique with the aim to generate high numbers of phenotypically, functionally and genetically defined early EPCs from human cord blood. Characterization of EPCs was done by flow cytometry, immunofluorescence microscopy, colony forming unit (CFU) assay and endothelial tube formation assay. There was an average 48-fold increase in EPC numbers. EPCs expressed VEGFR-2, CD144, CD18, and CD61, and were positive for acetylated LDL uptake and ulex lectin binding. The cells stimulated endothelial tube formation only in co-cultures with mature endothelial cells and formed CFUs. Microarray analysis revealed highly up-regulated genes, including LL-37 (CAMP), PDK4, and alpha-2-macroglobulin. In addition, genes known to be associated with cardioprotective (GDF15) or pro-angiogenic (galectin-3) properties were also significantly up-regulated after a 72 h differentiation period on fibronectin. We present a novel method that allows to generate high numbers of phenotypically, functionally and genetically characterized early EPCs. Furthermore, we identified several genes newly linked to EPC differentiation, among them LL-37 (CAMP) was the most up-regulated gene.

Identification of integrin beta subunit mutations that alter affinity for extracellular matrix ligand

We examined over 50 mutations in the Drosophila βPS integrin subunit that alter integrin function in situ for their ability to bind a soluble monovalent ligand, TWOW-1. Surprisingly, very few of the mutations, which were selected for conditional lethality in the fly, reduce the ligand binding ability of the integrin. The most prevalent class of mutations activates the integrin heterodimer. These findings emphasize the importance of integrin affinity regulation and point out how molecular interactions throughout the integrin molecule are important in keeping the integrin in a low affinity state. Mutations strongly support the controversial deadbolt hypothesis, where the CD loop in the β tail domain acts to restrain the I domain in the inactive, bent conformation. Site-directed mutations in the cytoplasmic domains of βPS and αPS2C reveal different effects on ligand binding from those observed for αIIbβ3 integrins and identify for the first time a cytoplasmic cysteine residue, conserved in three human integrins, as being important in affinity regulation. In the fly, we find that genetic interactions of the βPS mutations with reduction in talin function are consistent with the integrin affinity differences measured in cells. Additionally, these genetic interactions report on increased and decreased integrin functions that do not result in affinity changes in the PS2C integrin measured in cultured cells.

Genetic transfer of fusion proteins effectively inhibits VCAM-1-mediated cell adhesion and transmigration via inhibition of cytoskeletal anchorage

The adhesion of leukocytes to endothelium plays a central role in the development of atherosclerosis and thus represents an attractive therapeutic target for anti-atherosclerotic therapies. Vascular cell adhesion molecule-1 (VCAM-1) mediates both the initial tethering and the firm adhesion of leukocytes to endothelial cells. Our work evaluates the feasibility of using the cytoskeletal anchorage of VCAM-1 as a target for gene therapy. As a proof of concept, integrin α_IIbβ₃-mediated cell adhesion with clearly defined cytoskeletal anchorage was tested. We constructed fusion proteins containing the intracellular domain of β₃ placed at various distances to the cell membrane. Using cell adhesion assays and immunofluorescence, we established fusion constructs with competitive and dominant negative inhibition of cell adhesion. With the goal being the transfer of the dominant negative mechanism towards VCAM-1 inhibition, we constructed a fusion molecule containing the cytoplasmic domain of VCAM-1. Indeed, VCAM-1 mediated leukocyte adhesion can be inhibited via transfection of DNA encoding the designed VCAM-1 fusion protein. This is demonstrated in adhesion assays under static and flow conditions using CHO cells expressing recombinant VCAM-1 as well as activated endothelial cells. Thus, we are able to describe a novel approach for dominant negative inhibition of leukocyte adhesion to endothelial cells. This approach warrants further development as a novel gene therapeutic strategy that aims for a locally restricted effect at atherosclerotic areas of the vasculature.

Studies of tricyclic piperazine/piperidine furnished molecules as novel integrin αvβ3/αIIbβ3 dual antagonists using 3D-QSAR and molecular docking

The development of injectable integrin α(v)β(3)/α(IIb)β(3) dual antagonists attracts much attention of research for treating of acute ischemic diseases in recent years. In this work, based on a dataset composed of 102 tricyclic piperazine/piperidine furnished dual α(v)β(3) and α(IIb)β(3) antagonists, a variety of in silico modeling approaches including the comparative molecular field analysis (CoMFA), comparative similarity indices analysis (CoMSIA), and molecular docking were applied to reveal the requisite 3D structural features impacting the biological activities. Our statistical results show that the ligand-based 3D-QSAR models for both the α(v)β(3) and α(IIb)β(3) studies exhibited satisfactory internal and external predictability, i.e., for the CoMFA models, results of Q(2)=0.48, R(ncv)(2)=0.87, R(pred)(2)=0.71 for α(v)β(3) and Q(2)=0.50, R(ncv)(2)=0.85, R(pred)(2)=0.72 for α(IIb)β(3) analysis were obtained, and for the CoMSIA ones, the outcomes of Q(2)=0.55, R(ncv)(2)=0.90, R(pred)(2)=0.72 for α(v)β(3) and Q(2)=0.52, R(ncv)(2)=0.88, R(pred)(2)=0.74 for α(IIb)β(3) were achieved respectively. In addition, through a comparison between 3D-QSAR contour maps and docking results, it is revealed that that the most crucial interactions occurring between the tricyclic piperazine/piperidine derivatives and α(v)β(3)/α(IIb)β(3) receptor ligand binding pocket are H-bonding, and the key amino acids impacting the interactions are Arg214, Asn215, Ser123, and Lys253 for α(v)β(3), but Arg214, Asn215, Ser123 and Tyr190 for α(IIb)β(3) receptors, respectively. Halogen-containing groups at position 15 and 16, benzene sulfonamide substituent at position 23, and the replacement of piperazine with 4-aminopiperidine of ring B may increase the α(v)β(3)/α(IIb)β(3) antagonistic activity. The potencies for antagonists to inhibit isolated α(v)β(3) and α(IIb)β(3) are linear correlated, indicating that similar interaction mechanisms may exist for the series of molecules. To our best knowledge this is the first report on 3D-QSAR modeling of these dual α(v)β(3)/α(IIb)β(3) antagonists. The results obtained should provide information for better understanding of the mechanism of antagonism and thus be helpful in design of novel potent dual α(v)β(3)/α(IIb)β(3) antagonists.

Targeted molecular dynamics reveals overall common conformational changes upon hybrid domain swing-out in beta3 integrins

The beta3 integrin family members alphaIIbeta3 and alphaVbeta3 signal bidirectionally through long-range allosteric changes, including a transition from a bent unliganded-closed low-affinity state to an extended liganded-open high-affinity state. To obtain an atomic-level description of this transition in an explicit solvent, we carried out targeted molecular dynamics simulations of the headpieces of alphaIIbeta3 and alphaVbeta3 integrins. Although minor differences were observed between these receptors, our results suggest a common transition pathway in which the hybrid domain swing-out is accompanied by conformational changes within the beta3 betaA (I-like) domain that propagate through the alpha7 helix C-terminus, and are followed by the alpha7 helix downward motion and the opening of the beta6-alpha7 loop. Breaking of contact interactions between the beta6-alpha7 loop and the alpha1 helix N-terminus results in helix straightening, internal rearrangements of the specificity determining loop (SDL), movement of the beta1-alpha1 loop toward the metal ion dependent adhesion site (MIDAS), and final changes at the interfaces between the beta3 betaA (I-like) domain and either the hybrid or the alpha beta-propeller domains. Taken together, our results suggest novel testable hypotheses of intradomain and interdomain interactions responsible for beta3 integrin activation.

Structure of an integrin alphaIIb beta3 transmembrane-cytoplasmic heterocomplex provides insight into integrin activation

Heterodimeric integrin adhesion receptors regulate diverse biological processes including angiogenesis, thrombosis and wound healing. The transmembrane-cytoplasmic domains (TMCDs) of integrins play a critical role in controlling activation of these receptors via an inside-out signaling mechanism, but the precise structural basis remains elusive. Here, we present the solution structure of integrin alphaIIb beta3 TMCD heterodimer, which reveals a right-handed coiled-coil conformation with 2 helices intertwined throughout the transmembrane region. The helices extend into the cytoplasm and form a clasp that differs significantly from a recently published alphaIIb beta3 TMCD structure. We show that while a point mutation in the clasp interface modestly activates alphaIIb beta3, additional mutations in the transmembrane interface have a synergistic effect, leading to extensive integrin activation. Detailed analyses and structural comparison with previous studies suggest that extensive integrin activation is a highly concerted conformational transition process, which involves transmembrane coiled-coil unwinding that is triggered by the membrane-mediated alteration and disengagement of the membrane-proximal clasp. Our results provide atomic insight into a type I transmembrane receptor heterocomplex and the mechanism of integrin inside-out transmembrane signaling.

Equine herpesvirus 1 entry via endocytosis is facilitated by alphaV integrins and an RSD motif in glycoprotein D

Equine herpesvirus 1 (EHV-1) is a member of the Alphaherpesvirinae, and its broad tissue tropism suggests that EHV-1 may use multiple receptors to initiate virus entry. EHV-1 entry was thought to occur exclusively through fusion at the plasma membrane, but recently entry via the endocytic/phagocytic pathway was reported for Chinese hamster ovary cells (CHO-K1 cells). Here we show that cellular integrins, and more specifically those recognizing RGD motifs such as alphaVbeta5, are important during the early steps of EHV-1 entry via endocytosis in CHO-K1 cells. Moreover, mutational analysis revealed that an RSD motif in the EHV-1 envelope glycoprotein D (gD) is critical for entry via endocytosis. In addition, we show that EHV-1 enters peripheral blood mononuclear cells predominantly via the endocytic pathway, whereas in equine endothelial cells entry occurs mainly via fusion at the plasma membrane. Taken together, the data in this study provide evidence that EHV-1 entry via endocytosis is triggered by the interaction between cellular integrins and the RSD motif present in gD and, moreover, that EHV-1 uses different cellular entry pathways to infect important target cell populations of its natural host.

Key interactions in integrin ectodomain responsible for global conformational change detected by elastic network normal-mode analysis

Integrin, a membrane protein with a huge extracellular domain, participates in cell-cell and cell-extracellular-matrix interactions for metazoan. A group of integrins is known to perform a large-scale structural change when the protein is activated, but the activation mechanism and generality of the conformational change remain to be elucidated. We performed normal-mode analysis of the elastic network model on integrin alpha(V)beta(3) ectodomain in the bent form and identified key residues that influenced molecular motions. Iterative normal-mode calculations demonstrated that the specific nonbonded interactions involving the key residues work as a snap to keep integrin in the bent form. The importance of the key residues for the conformational change was further verified by mutation experiments, in which integrin alpha(IIb)beta(3) was used. The conservation pattern of amino acid residues among the integrin family showed that the characteristic pattern of residues seen around these key residues is found in the limited groups of integrin beta-chains. This conservation pattern suggests that the molecular mechanism of the conformational change relying on the interactions found in integrin alpha(V)beta(3) is unique to the limited types of integrins.

Fibrinogen interaction of CHO cells expressing chimeric alphaIIb/alphavbeta3 integrin

AIM

The molecular mechanisms of the affinity regulation of alphavbeta3 integrin are important in tumor development, wound repairing, and angiogenesis. It has been established that the cytoplasmic domains of alphavbeta3 integrin play an important role in integrin-ligand affinity regulation. However, the relationship of structure-function within these domains remains unclear.

METHODS

The extracellular and transmembrane domain of alphaIIb was fused to the alphav integrin cytoplasmic domain, and the chimeric alpha subunit was coexpressed in Chinese hamster ovary (CHO) cells with the wild-type beta3 subunit or with 3 mutant beta3 sequences bearing truncations at the positions of T741, Y747, and F754, respectively. The CHO cells expressing these recombinant integrins were tested for soluble fibrinogen binding and the cell adhesion and spreading on immobilized fibrinogen.

RESULTS

All 4 types of integrins bound soluble fibrinogen in the absence of agonist stimulation, and only the cells expressing the chimeric alpha subunit with the wild-type beta3 subunit, but not those with truncated beta3, could adhere to and spread on immobilized fibrinogen.

CONCLUSION

The substitution alphaIIb at the cytoplasmic domain with the alphav cytoplasmic sequence rendered the extracellular alphaIIbbeta3 a constitutively activated conformation for ligands without the need of pinside-outq signals. Our results also indicated that the COOH-terminal sequence of beta3 might play a key role in integrin alphaIIb/alphavbeta3-mediated cell adhesion and spreading on immobilized fibrinogen. The cells expressing alphaIIb/alphavbeta3 have enormous potential for facilitating drug screening for antagonists either to alphavbeta3 intracellular interactions or to alphaIIbbeta3 receptor functions.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

Generation of activation-specific human anti-alphaMbeta2 single-chain antibodies as potential diagnostic tools and therapeutic agents

The leukocyte integrin Mac-1 (alpha(M)beta(2)) plays a pivotal role in inflammation and host defense. Upon leukocyte activation, Mac-1 undergoes a conformational change exposing interaction sites for multiple ligands. We aimed to generate single-chain antibodies (scFv's) directed against activation-specific Mac-1 ligand-binding sites. Using human scFv phage libraries, we developed subtractive strategies with depletion of phages binding to nonactivated Mac-1 and selection of phages binding to activated Mac-1, using monocytes as well as CHO cells transfected with native or mutated, activated Mac-1. Three scFv clones demonstrated exclusive binding to activated Mac-1. Mac-1 binding of the ligands fibrinogen, heparin, and ICAM-1, but not C3bi, was inhibited. Using alanine substitutions, the paratope was identified within the heavy chain HCDR3s of the scFv's. The epitope was localized to Lys(245)-Arg(261) of the alpha(M) I-domain. In a pilot study with septicemic patients, we provide initial support for the use of these scFv's as markers of monocyte activation and as potential diagnostic tools. Potential therapeutic use was tested in adhesion assays under static and flow conditions demonstrating the selective blockade of activated monocytes only. Furthermore, scFv HCDR3-derived peptides retain selectivity for the activated integrin, providing a unique template for the potential development of inhibitors that are specific for the activated Mac-1.

Integrin signaling is critical for pathological angiogenesis

The process of postnatal angiogenesis plays a crucial role in pathogenesis of numerous diseases, including but not limited to tumor growth/metastasis, diabetic retinopathy, and in tissue remodeling upon injury. However, the molecular events underlying this complex process are not well understood and numerous issues remain controversial, including the regulatory function of integrin receptors. To analyze the role of integrin phosphorylation and signaling in angiogenesis, we generated knock-in mice that express a mutant beta3 integrin unable to undergo tyrosine phosphorylation. Two distinct models of pathological angiogenesis revealed that neovascularization is impaired in mutant beta3 knock-in mice. In an ex vivo angiogenesis assay, mutant beta3 knock-in endothelial cells did not form complete capillaries in response to vascular endothelial growth factor (VEGF) stimulation. At the cellular level, defective tyrosine phosphorylation in mutant beta3 knock-in cells resulted in impaired adhesion, spreading, and migration of endothelial cells. At the molecular level, VEGF stimulated complex formation between VEGF receptor-2 and beta3 integrin in wild-type but not in mutant beta3 knock-in endothelial cells. Moreover, phosphorylation of VEGF receptor-2 was significantly reduced in cells expressing mutant beta3 compared to wild type, leading to impaired integrin activation in these cells. These findings provide novel mechanistic insights into the role of integrin-VEGF axis in pathological angiogenesis.

A novel functional role for the highly conserved alpha-subunit KVGFFKR motif distinct from integrin alphaIIbbeta3 activation processes

BACKGROUND

The highly conserved integrin alpha-subunit membrane-proximal motif KVGFFKR plays a decisive role in modulating the activation of integrin alphaIIbbeta3. Previously, we have shown that a platelet permeable palmityl (pal)-peptide with this seven amino acid sequence can directly activate alphaIIbbeta3 leading to platelet aggregation.

OBJECTIVES

To investigate further the role of the KVGFFKR motif in integrin alphaIIbbeta3 function.

METHODS

We used two sequence-specific complementary model systems, palmityl pal-peptides in platelets, and mutant alphaIIbbeta3-expressing Chinese Hamster Ovary (CHO) cell lines.

RESULTS

In platelets we show that the two phenylalanine amino acids in pal-KVGFFKR (pal-FF) peptide are critical for stimulating platelet aggregation. Pal-FF peptide treatment of platelets also gives rise to a tyrosine phosphorylation signal despite the presence of inhibitors of fibrinogen binding. In CHO cells, a double alanine substitution, alphaIIb(F992A, F993A)beta3, induces constitutive integrin activation but prevents actin stress fiber formation upon adhesion to fibrinogen, suggesting that alphaIIbbeta3-mediated cytoskeletal reorganization is also dependent on F992 and F993. This further highlights a critical role for the two phenylalanine residues in both of these alphaIIbbeta3-mediated processes.

CONCLUSION

In addition to regulating integrin alphaIIbbeta3 activation state, the KVGFFKR motif also influences cytoskeletal reorganization. This activity is critically determined by F992 and F993 within the seven amino acid sequence.