Breaking the integrin hinge. A defined structural constraint regulates integrin signaling

The roles of platelet GPIIb/IIIa and alphavbeta3 integrins during HeLa cells adhesion, migration, and invasion to monolayer endothelium under static and dynamic shear flow

During their passage through the circulatory system, tumor cells undergo extensive interactions with various host cells including endothelial cells and platelets. Mechanisms mediating tumor cell adhesion, migration, and metastasis to vessel wall under flow condition are largely unknown. The aim of this study was to investigate the potential roles of GPIIb/IIIa and αvβ3 integrins underlying the HeLa-endothelium interaction in static and dynamic flow conditions. HeLa cell migration and invasion were studied by using Millicell cell culture insert system. The numbers of transmigrated or invaded HeLa cells significantly increased by thrombin-activated platelets and reduced by eptifibatide, a platelet inhibitor. Meanwhile, RGDWE peptides, a specific inhibitor of αvβ3 integrin, also inhibited HeLa cell transmigration. Interestingly, the presence of endothelial cells had significant effect on HeLa cell migration regardless of static or cocultured flow condition. The adhesion capability of HeLa cells to endothelial monolayer was also significantly affected by GPIIb/IIIa and αvβ3 integrins. The arrested HeLa cells increased nearly 5-fold in the presence of thrombin-activated platelets at shear stress condition (1.84 dyn/cm² exposure for 1 hour) than the control (static). Our findings showed that GPIIb/IIIa and αvβ3 integrins are important mediators in the pathology of cervical cancer and provide a molecular basis for the future therapy, and the efficient antitumor benefit should target multiple receptors on tumor cells and platelets.

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.

The structure of an integrin/talin complex reveals the basis of inside-out signal transduction

Fundamental to cell adhesion and migration, integrins are large heterodimeric membrane proteins that uniquely mediate inside-out signal transduction, whereby adhesion to the extracellular matrix is activated from within the cell by direct binding of talin to the cytoplasmic tail of the beta integrin subunit. Here, we report the first structure of talin bound to an authentic full-length beta integrin tail. Using biophysical and whole cell measurements, we show that a specific ionic interaction between the talin F3 domain and the membrane-proximal helix of the beta tail disrupts an integrin alpha/beta salt bridge that helps maintain the integrin inactive state. Second, we identify a positively charged surface on the talin F2 domain that precisely orients talin to disrupt the heterodimeric integrin transmembrane (TM) complex. These results show key structural features that explain the ability of talin to mediate inside-out TM signalling.

Calreticulin-independent regulation of the platelet integrin αIIbβ3by the KVGFFKR αIIb-cytoplasmic motif

The platelet integrin alphaIIbbeta3 alters conformation in response to platelet activation and ligand binding, although the molecular mechanisms involved are not known. We previously showed that a lipid modified peptide, corresponding to the membrane proximal 989KVGFFKR995 portion of the alphaIIb cytoplasmic tail, independently activates platelet alphaIIbbeta3. Calreticulin (CRT) is a potential integrin regulatory protein based on its interaction with the highly conserved alpha-integrin sequence KxGFFKR. We therefore examined the possible interaction of calreticulin and alphaIIbbeta3 in human platelets. We demonstrate that calreticulin in platelets is localised to the granulomere. In contrast, the known integrin-binding protein talin accumulates at the periphery of spreading platelets and colocalises with alphaIIbbeta3 during the process of adhesion. An interaction between calreticulin and alphaIIbbeta3 could not be demonstrated using co-immunoprecipitation techniques under various platelet activation states, even in the presence of covalent chemical crosslinkers. Thus, calreticulin does not functionally interact with the major integrin in human platelets. In order to identify proteins that interact with the integrin KVGFFKR motif we then used a peptide 'pull-down' assay from platelet lysates with biotinylated peptides and demonstrate that only the alphaIIb and beta3 subunits selectively and individually interact with this sequence. This interaction is divalent cation-dependent, has high-affinity, and occurs both with purified alphaIIbbeta3 complex and with electroeluted alpha and beta subunits. Thus, our data show that the conserved integrin KVGFFKR domain interacts primarily with the alpha and beta cytoplasmic tails and not with CRT in human platelets.

Overexpression of the partially activated alpha(IIb)beta3D723H integrin salt bridge mutant downregulates RhoA activity and induces microtubule-dependent proplatelet-like extensions in Chinese hamster ovary cells

BACKGROUND

We have recently reported a novel mutation in the beta3 subunit of the platelet fibrinogen receptor (alpha(IIb)beta3D723H) identified in a patient with dominantly inherited macrothrombocytopenia, and we have shown that this mutation promotes a new phenotype in Chinese hamster ovary (CHO) cells, characterized by fibrinogen-dependent, microtubule-driven proplatelet-like cell extensions.

RESULTS

Here we demonstrate that the partially activated alpha(IIb)beta3D723H or alpha(IIb)beta3D723A salt bridge mutants, but not fully activated alpha(IIb)beta3 mutants, cause this phenotype. Time-lapse videomicroscopy clearly differentiated these stable microtubule-driven and nocodazole-sensitive extensions from common dynamic actin-driven pseudopodia. In addition, overexpression of a mitochondrial marker confirmed their functional role in organelle transport. Comparative immunofluorescence analysis of the subcellular localization of alpha(IIb)beta3, the focal adhesion proteins talin or vinculin and actin revealed a similar membrane labeling of CHO cell extensions and CD34+-derived megakaryocyte proplatelets. Mutant alpha(IIb)beta3D723H signaling was independent of Src, protein kinase C or phosphoinositide 3-kinase, but correlated with decreased RhoA activity as compared with wild-type alpha(IIb)beta3 signaling, reminiscent of integrin signaling during neurite outgrowth. Accordingly, overexpression of constitutively active RhoA in CHO alpha(IIb)beta3D723H cells prevented protrusion formation on fibrinogen. Most interestingly, RhoA/ROCK inhibition was necessary, but not sufficient, and integrin activity was additionally required to induce CHO cell extension formation.

CONCLUSIONS

CHO alpha(IIb)beta3D723H cell protrusions and megakaryocyte proplatelets, like neuronal cell neurites, result from a common integrin-dependent signaling pathway, promoting strongly decreased RhoA activity and leading to microtubule-driven formation of cytoplasmic extensions.

The structure and function of platelet integrins

Integrins are a ubiquitous family of non-covalently associated alpha/beta transmembrane heterodimers linking extracellular ligands to intracellular signaling pathways [1] [Cell, 2002; 110: 673]. Platelets contain five integrins, three beta1 integrins that mediate platelet adhesion to the matrix proteins collagen, fibronectin and laminin, and the beta3 integrins alphavbeta3 and alphaIIbbeta3 [2] [J Clin Invest, 2005; 115: 3363]. While there are only several hundred alphavbeta3 molecules per platelet, alphavbeta3 mediates platelet adhesion to osteopontin and vitronectin in vitro [3] [J Biol Chem, 1997; 272: 8137]; whether this occurs in vivo remains unknown. By contrast, the 80,000 alphaIIbbeta3 molecules on agonist-stimulated platelets bind fibrinogen, von Willebrand factor, and fibronectin, mediating platelet aggregation when the bound proteins crosslink adjacent platelets [2] [J Clin Invest, 2005; 115: 3363]. Although platelet integrins are poised to shift from resting to active conformations, tight regulation of their activity is essential to prevent the formation of intravascular thrombi. This review focuses on the structure and function of the intensively studied beta3 integrins, in particular alphaIIbbeta3, but reference will be made to other integrins where relevant.

The structure of a receptor with two associating transmembrane domains on the cell surface: integrin alphaIIbbeta3

Structures of intact receptors with single-pass transmembrane domains are essential to understand how extracellular and cytoplasmic domains regulate association and signaling through transmembrane domains. A chemical and computational method to determine structures of the membrane regions of such receptors on the cell surface is developed here and validated with glycophorin A. An integrin heterodimer structure reveals association over most of the lengths of the alpha and beta transmembrane domains and shows that the principles governing association of hetero and homo transmembrane dimers differ. A turn at the Gly of the juxtamembrane GFFKR motif caps the alpha TM helix and brings the two Phe of GFFKR into the alpha/beta interface. A juxtamembrane Lys residue in beta also has an important role in the interface. The structure shows how transmembrane association/dissociation regulates integrin signaling. A joint ectodomain and membrane structure shows that substantial flexibility between the extracellular and TM domains is compatible with TM signaling.

Activation of tumor cell integrin alphavbeta3 controls angiogenesis and metastatic growth in the brain

The incidence of brain metastasis is rising and poses a severe clinical problem, as we lack effective therapies and knowledge of mechanisms that control metastatic growth in the brain. Here we demonstrate a crucial role for high-affinity tumor cell integrin alpha(v)beta(3) in brain metastatic growth and recruitment of blood vessels. Although alpha(v)beta(3) is frequently up-regulated in primary brain tumors and metastatic lesions of brain homing cancers, we show that it is the alpha(v)beta(3) activation state that is critical for brain lesion growth. Activated, but not non-activated, tumor cell alpha(v)beta(3) supports efficient brain metastatic growth through continuous up-regulation of vascular endothelial growth factor (VEGF) protein under normoxic conditions. In metastatic brain lesions carrying activated alpha(v)beta(3), VEGF expression is controlled at the post-transcriptional level and involves phosphorylation and inhibition of translational respressor 4E-binding protein (4E-BP1). In contrast, tumor cells with non-activated alpha(v)beta(3) depend on hypoxia for VEGF induction, resulting in reduced angiogenesis, tumor cell apoptosis, and inefficient intracranial growth. Importantly, the microenvironment critically influences the effects that activated tumor cell alpha(v)beta(3) exerts on tumor cell growth. Although it strongly promoted intracranial growth, the activation state of the receptor did not influence tumor growth in the mammary fat pad as a primary site. Thus, we identified a mechanism by which metastatic cells thrive in the brain microenvironment and use the high-affinity form of an adhesion receptor to grow and secure host support for proliferation. Targeting this molecular mechanism could prove valuable for the inhibition of brain metastasis.

Multiple approaches converge on the structure of the integrin alphaIIb/beta3 transmembrane heterodimer

Integrins link the cytoskeleton to the extracellular matrix and regulate key signaling events that coordinate cellular processes such as secretion, migration, and proliferation. A single integrin molecule can exist in a resting state that does not bind extracellular ligands or in an active state that can engage ligands and form large signaling complexes. Activation signals are transduced between the cytosolic region and the extracellular region by a binary on/off switch in the integrin's transmembrane (TM) domain; the integrin's alpha and beta subunits each have a single TM helix that forms an alpha/beta heterodimer in the resting state, and the TM heterodimer separates to transduce an activation signal across the membrane. In this article, two methods used to generate models of the TM heterodimer, both converging on the same structure, are described. The first model was generated by a Monte Carlo algorithm that selected conformations based on their agreement with published experimental mutagenesis results. The second model was generated by threading the integrin's sequence onto TM helix dimers parsed from the Protein Data Bank and by selecting conformations based on their agreement with published experimental cysteine crosslinking results. The two models have similar structures; however, they differ markedly from some previously published models. To distinguish conformations that reflect the native integrin, we compared the Monte Carlo model, the threaded model, and four published models with experimental mutagenesis and cysteine crosslinking results. The models presented here had high correlation coefficients when compared with experimental findings, and they are in excellent agreement, both in terms of accuracy and in terms of precision, with a recent NMR structure. These results demonstrate that multiple approaches converged on the same structure of the resting integrin's TM heterodimer, and this conformation likely reflects the integrin's native structure.

The tail of integrins, talin, and kindlins

Integrins are transmembrane cell-adhesion molecules that carry signals from the outside to the inside of the cell and vice versa. Like other cell surface receptors, integrins signal in response to ligand binding; however, events within the cell can also regulate the affinity of integrins for ligands. This feature is important in physiological situations such as those in blood, in which cells are always in close proximity to their ligands, yet cell-ligand interactions occur only after integrin activation in response to specific external cues. This review focuses on the mechanisms whereby two key proteins, talin and the kindlins, regulate integrin activation by binding the tails of integrin-beta subunits.

The structure of the integrin alphaIIbbeta3 transmembrane complex explains integrin transmembrane signalling

Heterodimeric integrin adhesion receptors regulate cell migration, survival and differentiation in metazoa by communicating signals bi-directionally across the plasma membrane. Protein engineering and mutagenesis studies have suggested that the dissociation of a complex formed by the single-pass transmembrane (TM) segments of the alpha and beta subunits is central to these signalling events. Here, we report the structure of the integrin alphaIIbbeta3 TM complex, structure-based site-directed mutagenesis and lipid embedding estimates to reveal the structural event that underlies the transition from associated to dissociated states, that is, TM signalling. The complex is stabilized by glycine-packing mediated TM helix crossing within the extracellular membrane leaflet, and by unique hydrophobic and electrostatic bridges in the intracellular leaflet that mediate an unusual, asymmetric association of the 24- and 29-residue alphaIIb and beta3 TM helices. The structurally unique, highly conserved integrin alphaIIbbeta3 TM complex rationalizes bi-directional signalling and represents the first structure of a heterodimeric TM receptor complex.

Ancient ubiquitous protein 1 and Syk link cytoplasmic tails of the integrin alpha(IIb)beta(3)

It is currently accepted that activity of the integrin alpha(IIb)beta(3) is modulated by direct interaction between its cytoplasmic tails (CTs) and association of cytoplasmic proteins with them, and disruption of the close linkage between CTs leads to activation of alpha(IIb)beta(3) (inside-out signaling). We previously reported that ancient ubiquitous protein (Aup1) binds to the membrane-proximal sequence of integrin alphaCTs that plays a pivotal role in the inside-out signaling. To explore biological function of Aup1, we examined in this study interaction of Aup1 with Src and Syk that are quickly activated in platelets before fibrinogen binding following thrombin stimulation. By immunoprecipitation assay with resting platelets, we first found that alpha(IIb)beta(3), Src, Syk, and Aup1 are constitutively complexed. In vitro binding study with recombinant Syk and glutathione (GSH) S-transferase (GST) - Src, -Aup1, and -alpha(IIb) and - beta(3) CTs that are immobilized to GSH- beads revealed direct binding of Syk to Aup1 as well as the beta(3) CT. Dot blot analysis with synthetic peptides for alpha(IIb) and beta(3) CTs, and GST-Aup1 and -Src immobilized to PVDF membrane exhibited concordant result with the GST pull-down assay. Immunoprecipitation of platelet lysates 10 seconds after thrombin stimulation, when activity and tyrosine phosphorylation of Syk are maimum, exhibited that active Syk does not coprecipitate with Aup1. In vitro kinase assay with GST-Syk and -Aup1 proteins at the presence or absence of active Src, a potent activator of Syk, revealed that Aup1 does not directly influence activation of Syk by autophosphorylation or tyrosine phosphorylation by Src. These results indicate that Aup1 is an adaptor recruiting Syk to the alpha(IIb) CT, and suggest that the alpha(IIb) -Aup1- Syk- beta(3) complex formation links alpha(IIb) and beta(3) CTs to sustain alpha(IIb)beta(3) in an inactive state and Syk dissociates from Aup1 after activation.

Regulation of integrin activity and signalling

The ability of cells to attach to each other and to the extracellular matrix is of pivotal significance for the formation of functional organs and for the distribution of cells in the body. Several molecular families of proteins are involved in adhesion, and recent work has substantially improved our understanding of their structures and functions. Also, these molecules are now being targeted in the fight against disease. However, less is known about how their activity is regulated. It is apparent that among the different classes of adhesion molecules, the integrin family of adhesion receptors is unique in the sense that they constitute a large group of widely distributed receptors, they are unusually complex and most importantly their activities are strictly regulated from the inside of the cell. The activity regulation is achieved by a complex interplay of cytoskeletal proteins, protein kinases, phosphatases, small G proteins and adaptor proteins. Obviously, we are only in the beginning of our understanding of how the integrins function, but already now fascinating details have become apparent. Here, we describe recent progress in the field, concentrating mainly on mechanistical and structural studies of integrin regulation. Due to the large number of articles dealing with integrins, we focus on what we think are the most exciting and rewarding directions of contemporary research on cell adhesion and integrins.

Linking integrin conformation to function

Integrins are alphabeta heterodimeric adhesion receptors that relay signals bidirectionally across the plasma membrane between the extracellular matrix and cell-surface ligands, and cytoskeletal and signalling effectors. The physical and chemical signals that are controlled by integrins are essential for intercellular communication and underpin all aspects of metazoan existence. To mediate such diverse functions, integrins exhibit structural diversity, flexibility and dynamism. Conformational changes, as opposed to surface expression or clustering, are central to the regulation of receptor function. In recent years, there has been intense interest in determining the three-dimensional structure of integrins, and analysing the shape changes that underpin the interconversion between functional states. Considering the central importance of the integrin signalling nexus, it is perhaps no surprise that obtaining this information has been difficult, and the answers gained so far have been complicated. In this Commentary, we pose some of the key remaining questions that surround integrin structure-function relationships and review the evidence that supports the current models.

Mechanisms that regulate adaptor binding to beta-integrin cytoplasmic tails

Cells recognize and respond to their extracellular environment through transmembrane receptors such as integrins, which physically connect the extracellular matrix to the cytoskeleton. Integrins provide the basis for the assembly of intracellular signaling platforms that link to the cytoskeleton and influence nearly every aspect of cell physiology; however, integrins possess no enzymatic or actin-binding activity of their own and thus rely on adaptor molecules, which bind to the short cytoplasmic tails of integrins, to mediate and regulate these functions. Many adaptors compete for relatively few binding sites on integrin tails, so regulatory mechanisms have evolved to reversibly control the spatial and temporal binding of specific adaptors. This Commentary discusses the adaptor proteins that bind directly to the tails of beta integrins and, using talin, tensin, filamin, 14-3-3 and integrin-linked kinase (ILK) as examples, describes the ways in which their binding is regulated.

Mislocalized scaffolding by the Na-H exchanger NHE1 dominantly inhibits fibronectin production and TGF-beta activation

Secretion and assembly of the extracellular matrix protein fibronectin regulates a number of normal cell and tissue functions and is dysregulated in disease states such as fibrosis, diabetes, and cancer. We found that mislocalized scaffolding by the plasma membrane Na-H exchanger NHE1 suppresses fibronectin expression, secretion, and assembly. In fibroblasts, wild-type NHE1 localizes to the distal margin of membrane protrusions or lamellipodia but a mutant NHE1-KRA2 lacking binding sites for PI(4,5)P2 and the ERM proteins ezrin, radixin, and moesin is mislocalized and found uniformly along the plasma membrane. Although NHE1 regulates intracellular pH homeostasis, fibronectin production is not regulated by changes in intracellular pH, nor is it attenuated in NHE1-deficient cells, indicating fibronectin expression is independent of NHE1 activity. However, fibronectin production is nearly absent in cells expressing NHE1-KRA2 because scaffolding by NHE1 is mislocalized. Additionally, secretion of active but not latent TGF-beta is reduced and exogenous TGF-beta restores fibronectin secretion and assembly. Our data indicate that scaffolding by NHE1-KRA2 dominantly suppresses fibronectin synthesis and TGF-beta activation, and they suggest that NHE1-KRA2 can be used for obtaining a mechanistic understanding of how fibronectin production is regulated and speculatively for therapeutic control of dysregulated production in pathological conditions.

Migfilin, a molecular switch in regulation of integrin activation

The linkage of heterodimeric (alpha/beta) integrin receptors with their extracellular matrix ligands and intracellular actin cytoskeleton is a fundamental step for controlling cell adhesion and migration. Binding of the actin-linking protein, talin, to integrin beta cytoplasmic tails (CTs) induces high affinity ligand binding (integrin activation), whereas binding of another actin-linking protein, filamin, to the integrin beta CTs negatively regulates this process by blocking the talin-integrin interaction. Here we show structurally that migfilin, a novel cytoskeletal adaptor highly enriched in the integrin adhesion sites, strongly interacts with the same region in filamin where integrin beta CTs bind. We further demonstrate that the migfilin interaction dissociates filamin from integrin and promotes the talin/integrin binding and integrin activation. Migfilin thus acts as a molecular switch to disconnect filamin from integrin for regulating integrin activation and dynamics of extracellular matrix-actin linkage.

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Clustering and occupancy of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin alpha and beta transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, we develop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin alpha(IIb)beta(3) TMD tails via specific TMD interactions in mammalian cell membranes in lipid bicelles. Furthermore, charge reversal mutations at alpha(IIb)(R995)beta(3)(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the beta(3) tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin alpha(IIb)beta(3) TMD tails in mammalian cell membranes and document their role in integrin signaling.

Adhesion-induced unclasping of cytoplasmic tails of integrin alpha(IIb)beta3

Integrin alpha(IIb)beta(3) plays a pivotal role in hemostasis and thrombosis by mediating adhesive interactions of platelets. Binding of alpha(IIb)beta(3) to its physiological ligands, immobilized fibrinogen and fibrin, induces outside-in signaling in platelets, leading to their adhesion and spreading even without prior stimulation by agonists. Implicit in these phenomena is a requirement for the linkage between integrins' cytoplasmic tails and intracellular proteins. However, the nature of the initiating signal has not been established. In this study, we examined whether binding of alpha(IIb)beta(3) to immobilized fibrin(ogen), per se, triggers interaction of the integrin with cytoplasmic proteins. Using the integrin-binding skelemin fragment as a marker of exposure of residues involved in the clasp between alpha(IIb) and beta(3) cytoplasmic tails, we showed that its binding site in the membrane-proximal beta(3) 715-730 segment is cryptic and becomes exposed as a result of binding of isolated alpha(IIb)beta(3) to immobilized ligands. Furthermore, the skelemin-like protein present in platelets and CHO cells does not associate with alpha(IIb)beta(3) in resting platelets or suspended alpha(IIb)beta(3)-expressing CHO cells but is recruited to integrin during cell adhesion. In addition, not only beta(3) but also the membrane-proximal 989-1000 segment of the alpha(IIb) cytoplasmic tail binds the skelemin fragment. Finally, the same residues, alpha(IIb) Val(990), alpha(IIb) Arg(995), and beta(3) His(722), involved in the formation of the clasp between the tails are also required for skelemin binding. These studies suggest that ligation of alpha(IIb)beta(3) by immobilized ligands during platelet adhesion induces a transmembrane conformation change in the integrin, resulting in unclasping of the complex between the membrane-proximal parts of cytoplasmic tails, thereby unmasking residues involved in binding the skelemin-like protein. Thus, the junction between alpha(IIb) and beta(3) cytoplasmic tails may contain the critical structural information for the initiation of outside-in signaling.

A palmitoylated peptide, derived from the acidic carboxyl-terminal segment of the integrin alphaIIb cytoplasmic domain, inhibits platelet activation

Platelet integrin alpha(IIb)beta(3) contains an acidic membrane distal motif, 1000LEEDDEEGE1008, in the cytoplasmic domain of the alpha(IIb) subunit. We showed that a lipid-modified peptide corresponding to the above region, palmitoyl-K-LEEDDEEGE (pal-K-1000-1008), is platelet permeable and has inhibited platelet aggregation induced by 0.4 U/ml of thrombin (IC50 = 164 microM). Moreover the peptide inhibited both Fibrinogen and PAC-1, binding to activated platelets. The non palmitoylated analog was inactive. A modified, scrambled acidic peptide (palmitoyl-K-GDDEELEEE), showed significant lower inhibitory activity than pal-K-1000-1008. A palmitoylated peptide corresponding to the membrane proximal cytoplasmic domain of alpha(IIb), 989KGVFFKR995 (pal-989-995), is known to specifically induce platelet aggregation. Pal-K-1000-1008 was an inhibitor of human washed platelet aggregation induced by pal-K-989-995 (IC50 = 15 microM). Moreover, pal-K-1000-1008 inhibited phosphorylation of ERK and FAK, two protein kinases involved in platelet activation and aggregation. Our results favour the assumption that the interaction of the membrane proximal sequence 989KGVFFKR995 of the cytoplasmic domain of alpha(IIb) with the acidic terminal 1000LEEDDEEGE1008 motif may be an important structural factor in platelet signaling, leading to platelet activation and aggregation.

Display of cell surface sites for fibronectin assembly is modulated by cell adherence to (1)F3 and C-terminal modules of fibronectin

BACKGROUND

Fibronectin-null cells assemble soluble fibronectin shortly after adherence to a substrate coated with intact fibronectin but not when adherent to the cell-binding domain of fibronectin (modules (7)F3-(10)F3). Interactions of adherent cells with regions of adsorbed fibronectin other than modules (7)F3-(10)F3, therefore, are required for early display of the cell surface sites that initiate and direct fibronectin assembly.

METHODOLOGY/PRINCIPAL FINDINGS

To identify these regions, coatings of proteolytically derived or recombinant pieces of fibronectin containing modules in addition to (7)F3-(10)F3 were tested for effects on fibronectin assembly by adherent fibronectin-null fibroblasts. Pieces as large as one comprising modules (2)F3-(14)F3, which include the heparin-binding and cell adhesion domains, were not effective in supporting fibronectin assembly. Addition of module (1)F3 or the C-terminal modules to modules (2)F3-(14)F3 resulted in some activity, and addition of both (1)F3 and the C-terminal modules resulted in a construct, (1)F3-C, that best mimicked the activity of a coating of intact fibronectin. Constructs (1)F3-C V0, (1)F3-C V64, and (1)F3-C Delta(V(15)F3(10)F1) were all able to support fibronectin assembly, suggesting that (1)F3 through (11)F1 and/or (12)F1 were important for activity. Coatings in which the active parts of (1)F3-C were present in different proteins were much less active than intact (1)F3-C.

CONCLUSIONS

These results suggest that (1)F3 acts together with C-terminal modules to induce display of fibronectin assembly sites on adherent cells.

Kindlins: essential regulators of integrin signalling and cell-matrix adhesion

Integrin-mediated cell-ECM (extracellular matrix) adhesion is a fundamental process that controls cell behaviour. For correct cell-ECM adhesion, both the ligand-binding affinity and the spatial organization of integrins must be precisely controlled; how integrins are regulated, however, is not completely understood. Kindlins constitute a family of evolutionarily conserved cytoplasmic components of cell-ECM adhesions that bind to beta-integrin cytoplasmic tails directly and cooperate with talin in integrin activation. In addition, kindlins interact with many components of cell-ECM adhesions--such as migfilin and integrin-linked kinase--to promote cytoskeletal reorganization. Loss of kindlins causes severe defects in integrin signalling, cell-ECM adhesion and cytoskeletal organization, resulting in early embryonic lethality (kindlin-2), postnatal lethality (kindlin-3) and Kindler syndrome (kindlin-1). It is therefore clear that kindlins, together with several other integrin-proximal proteins, are essential for integrin signalling and cell-ECM adhesion regulation.

A naturally occurring extracellular alpha-beta clasp contributes to stabilization of beta3 integrins in a bent, resting conformation

Control of alphaIIb beta3 and alphav beta3 integrin activation is critical for cardiovascular homeostasis. Mutations that perturb association of integrin alpha and beta subunits in their transmembrane and cytoplasmic regions activate the integrin heterodimer, suggesting that a low-affinity or "off" conformation is the default state, likely corresponding to the bent conformation seen in the crystal structure of alphav beta3. In this bent structure, a segment of alphav (301-308) and beta3 (560-567) are juxtaposed. Here we provide evidence that these regions of alphav/alphaIIb and beta3 function as a novel extracellular clasp to restrain activation. Synthetic peptides representing the alphaIIb and beta3 clasp regions promote integrin activation as judged by cell adhesion, cell spreading, and exposure of epitopes for three beta3 LIBS antibodies. Mutation of the clasp region of alphav or beta3 results in a constitutively activated integrin, confirming the role of the extracellular clasp in restraining integrin activation. Molecular dynamics simulations of the alphav beta3 structure yield a refined model for the alphav beta3 clasp and provide plausible explanations for the effects of the activating mutations.

Genetic perturbation of the putative cytoplasmic membrane-proximal salt bridge aberrantly activates alpha(4) integrins

alpha(4) integrins play a pivotal role in leukocyte migration and tissue-specific homing. The ability of integrins to bind ligand is dynamically regulated by activation-dependent conformational changes triggered in the cytoplasmic domain. An NMR solution structure defined a putative membrane-proximal salt bridge between the alpha(IIb)beta(3) integrin cytoplasmic tails, which restrains integrins in their low-affinity state. However, the physiological importance of this salt bridge in alpha(4) integrin regulation remains to be elucidated. To address this question, we disrupted the salt bridge in murine germ line by mutating the conserved cytoplasmic arginine R(GFFKR) in alpha(4) integrins. In lymphocytes from knock-in mice (alpha(4)-R/A(GFFKR)), alpha(4)beta(1) and alpha(4)beta(7) integrins exhibited constitutively up-regulated ligand binding. However, transmigration of these cells across VCAM-1 and MAdCAM-1 substrates, or across endothelial monolayers, was reduced. Perturbed detachment of the tail appeared to cause the reduced cell migration of alpha(4)-R/A(GFFKR) lymphocytes. In vivo, alpha(4)-R/A(GFFKR) cells exhibited increased firm adhesion to Peyer patch venules but reduced homing to the gut. Our results demonstrate that the membrane-proximal salt bridge plays a critical role in supporting proper alpha(4) integrin adhesive dynamics. Loss of this interaction destabilizes the nonadhesive conformation, and thereby perturbs the properly balanced cycles of adhesion and deadhesion required for efficient cell migration.

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.

Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alphaIIbbeta3. PMA activated alphaIIbbeta3 only after expression of both PKCalpha (protein kinase Calpha) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCalpha. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alphaIIbbeta3 and bypassed the requirement for PKCalpha and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alphaIIbbeta3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alphaIIbbeta3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.

Transmembrane and cytoplasmic domains in integrin activation and protein-protein interactions (review)

Integrins are heterodimeric membrane-spanning adhesion receptors that are essential for a wide range of biological functions. Control of integrin conformational states is required for bidirectional signalling across the membrane. Key components of this control mechanism are the transmembrane and cytoplasmic domains of the alpha and beta subunits. These domains are believed to interact, holding the integrin in the inactive state, while inside-out integrin activation is accompanied by domain separation. Although there are strong indications for domain interactions, the majority of evidence is insufficient to precisely define the interaction interface. The current best model of the complex, derived from computational calculations with experimental restraints, suggests that integrin activation by the cytoplasmic protein talin is accomplished by steric disruption of the alpha/beta interface. Better atomic-level resolution structures of the alpha/beta transmembrane/cytoplasmic domain complex are still required for the resting state integrin to corroborate this. Integrin activation is also controlled by competitive interactions involving the cytoplasmic domains, particularly the beta-tails. The concept of the beta integrin tail as a focal adhesion interaction 'hub' for interactions and regulation is discussed. Current efforts to define the structure and affinity of the various complexes formed by integrin tails, and how these interactions are controlled, e.g. by phosphorylation and localization, are described.

Functional analysis of the cytoplasmic domain of the integrin {alpha}1 subunit in endothelial cells

Integrin alpha1beta1, the major collagen type IV receptor, is expressed by endothelial cells and plays a role in both physiologic and pathologic angiogenesis. Because the molecular mechanisms whereby this collagen IV receptor mediates endothelial cell functions are poorly understood, truncation and point mutants of the integrin alpha1 subunit cytoplasmic tail (amino acids 1137-1151) were generated and expressed into alpha1-null endothelial cells. We show that alpha1-null endothelial cells expressing the alpha1 subunit, which lacks the entire cytoplasmic tail (mutant alpha1-1136) or expresses all the amino acids up to the highly conserved GFFKR motif (mutant alpha1-1143), have a similar phenotype to parental alpha1-null cells. Pro(1144) and Leu(1145) were shown to be necessary for alpha1beta1-mediated endothelial cell proliferation; Lys(1146) for adhesion, migration, and tubulogenesis and Lys(1147) for tubulogenesis. Integrin alpha1beta1-dependent endothelial cell proliferation is primarily mediated by ERK activation, whereas migration and tubulogenesis require both p38 MAPK and PI3K/Akt activation. Thus, distinct amino acids distal to the GFFKR motif of the alpha1 integrin cytoplasmic tail mediate activation of selective downstream signaling pathways and specific endothelial cell functions.

The talin rod IBS2 alpha-helix interacts with the beta3 integrin cytoplasmic tail membrane-proximal helix by establishing charge complementary salt bridges

Talin establishes a major link between integrins and actin filaments and contains two distinct integrin binding sites: one, IBS1, located in the talin head domain and involved in integrin activation and a second, IBS2, that maps to helix 50 of the talin rod domain and is essential for linking integrin beta subunits to the cytoskeleton ( Moes, M., Rodius, S., Coleman, S. J., Monkley, S. J., Goormaghtigh, E., Tremuth, L., Kox, C., van der Holst, P. P., Critchley, D. R., and Kieffer, N. (2007) J. Biol. Chem. 282, 17280-17288 ). Through the combined approach of mutational analysis of the beta3 integrin cytoplasmic tail and the talin rod IBS2 site, SPR binding studies, as well as site-specific antibody inhibition experiments, we provide evidence that the integrin beta3-talin rod interaction relies on a helix-helix association between alpha-helix 50 of the talin rod domain and the membrane-proximal alpha-helix of the beta3 integrin cytoplasmic tail. Moreover, charge complementarity between the highly conserved talin rod IBS2 lysine residues and integrin beta3 glutamic acid residues is necessary for this interaction. Our results support a model in which talin IBS2 binds to the same face of the beta3 subunit cytoplasmic helix as the integrin alphaIIb cytoplasmic tail helix, suggesting that IBS2 can only interact with the beta3 subunit following integrin activation.

Structure of the integrin alphaIIb transmembrane segment

Integrin cell-adhesion receptors transduce signals bidirectionally across the plasma membrane via the single-pass transmembrane segments of each alpha and beta subunit. While the beta3 transmembrane segment consists of a linear 29-residue alpha-helix, the structure of the alphaIIb transmembrane segment reveals a linear 24-residue alpha-helix (Ile-966 -Lys-989) followed by a backbone reversal that packs Phe-992-Phe-993 against the transmembrane helix. The length of the alphaIIb transmembrane helix implies the absence of a significant transmembrane helix tilt in contrast to its partnering beta3 subunit. Sequence alignment shows Gly-991-Phe-993 to be fully conserved among all 18 human integrin alpha subunits, suggesting that their unusual structural motif is prototypical for integrin alpha subunits. The alphaIIb transmembrane structure demonstrates a level of complexity within the membrane that is beyond simple transmembrane helices and forms the structural basis for assessing the extent of structural and topological rearrangements upon alphaIIb-beta3 association, i.e. integrin transmembrane signaling.

Kindlin-2 (Mig-2): a co-activator of beta3 integrins

Integrin activation is essential for dynamically linking the extracellular environment and cytoskeletal/signaling networks. Activation is controlled by integrins' short cytoplasmic tails (CTs). It is widely accepted that the head domain of talin (talin-H) can mediate integrin activation by binding to two sites in integrin β's CT; in integrin β₃ this is an NPLY⁷⁴⁷ motif and the membrane-proximal region. Here, we show that the C-terminal region of integrin β₃ CT, composed of a conserved TS⁷⁵²T region and NITY⁷⁵⁹ motif, supports integrin activation by binding to a cytosolic binding partner, kindlin-2, a widely distributed PTB domain protein. Co-transfection of kindlin-2 with talin-H results in a synergistic enhancement of integrin α_IIbβ₃ activation. Furthermore, siRNA knockdown of endogenous kindlin-2 impairs talin-induced α_IIbβ₃ activation in transfected CHO cells and blunts α_vβ₃-mediated adhesion and migration of endothelial cells. Our results thus identify kindlin-2 as a novel regulator of integrin activation; it functions as a coactivator.

Differences in regulation of Drosophila and vertebrate integrin affinity by talin

Integrin-mediated cell adhesion is essential for development of multicellular organisms. In worms, flies, and vertebrates, talin forms a physical link between integrin cytoplasmic domains and the actin cytoskeleton. Loss of either integrins or talin leads to similar phenotypes. In vertebrates, talin is also a key regulator of integrin affinity. We used a ligand-mimetic Fab fragment, TWOW-1, to assess talin's role in regulating Drosophila alphaPS2 betaPS affinity. Depletion of cellular metabolic energy reduced TWOW-1 binding, suggesting alphaPS2 betaPS affinity is an active process as it is for vertebrate integrins. In contrast to vertebrate integrins, neither talin knockdown by RNA interference nor talin head overexpression had a significant effect on TWOW-1 binding. Furthermore, replacement of the transmembrane or talin-binding cytoplasmic domains of alphaPS2 betaPS with those of human alphaIIb beta3 failed to enable talin regulation of TWOW-1 binding. However, substitution of the extracellular and transmembrane domains of alphaPS2 betaPS with those of alphaIIb beta3 resulted in a constitutively active integrin whose affinity was reduced by talin knockdown. Furthermore, wild-type alphaIIb beta3 was activated by overexpression of Drosophila talin head domain. Thus, despite evolutionary conservation of talin's integrin/cytoskeleton linkage function, talin is not sufficient to regulate Drosophila alphaPS2 betaPS affinity because of structural features inherent in the alphaPS2 betaPS extracellular and/or transmembrane domains.

Transmembrane helices that form two opposite homodimeric interactions: an asparagine scan study of alphaM and beta2 integrins

Integrins are alpha/beta heterodimers, but recent in vitro and in vivo experiments also suggest an ability to associate through their transmembrane domains to form homomeric interactions. While the results of some in vitro experiments are consistent with an interaction mediated by a GxxxG-like motif, homo-oligomers observed after in vivo cross-linking are consistent with an almost opposite helix-helix interface. We have shown recently that both models of interaction are compatible with evolutionary conservation data, and we predicted that the alpha-helices in both models would have a similar rotational orientation. Herein, we have tested our prediction using in vitro asparagine scan of five consecutive residues along the GxxxG-like motif of the transmembrane domain of alpha and beta integrins, alphaM and beta2. We show that Asn-mediated dimerization occurs twice for every turn of the helix, consistent with two almost opposite forms of interaction as suggested previously for alphaIIb and beta3 transmembrane domains. The orientational parameters helix tilt and rotational orientation of each of these two Asn-stabilized dimers were measured by site-specific infrared dichroism (SSID) in model lipid bilayers and were found to be consistent with our predicted computational models. Our results highlight an intrinsic tendency for integrin transmembrane alpha-helices to form two opposite types of homomeric interaction in addition to their heteromeric interactions and suggest that integrins may form complex and specific networks at the transmembrane domain during function.

Protein phosphatase 2A negatively regulates integrin alpha(IIb)beta(3) signaling

Integrin alpha(IIb)beta(3) activation is critical for platelet physiology and is controlled by signal transduction through kinases and phosphatases. Compared with kinases, a role for phosphatases in platelet integrin alpha(IIb)beta(3) signaling is less understood. We report that the catalytic subunit of protein phosphatase 2A (PP2Ac) associates constitutively with the integrin alpha(IIb)beta(3) in resting platelets and in human embryonal kidney 293 cells expressing alpha(IIb)beta(3). The membrane proximal KVGFFKR sequence within the cytoplasmic domain of integrin alpha(IIb) is sufficient to support a direct interaction with PP2Ac. Fibrinogen binding to alpha(IIb)beta(3) during platelet adhesion decreased integrin-associated PP2A activity and increased the phosphorylation of a PP2A substrate, vasodilator associated phosphoprotein. Overexpression of PP2Ac(alpha) in 293 cells decreased alpha(IIb)beta(3)-mediated adhesion to immobilized fibrinogen. Conversely, small interference RNA mediated knockdown of endogenous PP2Ac(alpha) expression in 293 cells, enhanced extracellular signal-regulated kinase (ERK1/2) and p38 activation, and accelerated alpha(IIb)beta(3) adhesion to fibrinogen and von Willebrand factor. Inhibition of ERK1/2, but not p38 activation, abolished the increased adhesiveness of PP2Ac (alpha)-depleted 293 cells to fibrinogen. Furthermore, knockdown of PP2A(calpha) expression in bone marrow-derived murine megakaryocytes increased soluble fibrinogen binding induced by protease-activated receptor 4-activating peptide. These studies demonstrate that PP2Ac (alpha) can negatively regulate integrin alpha(IIb)beta(3) signaling by suppressing the ERK1/2 signaling pathway.

Identification of residues of functional importance within the central turn motifs present in the cytoplasmic tails of integrin alphaIIb and alphaV subunits

INTRODUCTION

Previous studies demonstrated that cell-permeable alphaIIb cytoplasmic peptides can modulate the activation of alphaIIbbeta3. An integrin activation motif was mapped to its membrane proximal region and a double proline mutant peptide and receptor indicated that its central turn motif had inhibitory capacity. However, the residues critical for inhibition of alphaIIbbeta3 activation were not identified. Using central turn peptides derived from alphaIIb and alphaV, residues critical for suppression of integrin activation were identified and the importance of these residues in protein-protein interactions was assessed.

MATERIALS AND METHODS

Cell-permeable peptides were used to determine the capacity of the central turn peptides to suppress alphaIIbbeta3 and alphaVbeta3 activation. Far Western analysis was used to characterize the capacity of the peptides to interact with CIB1 and surface plasmon resonance was used to characterize the binding of an antibody to the cytoplasmic tails of alphaIIb and alphaV.

RESULTS AND CONCLUSIONS

The central turn peptide from alphaV, alphaV(993-1001), has full inhibitory capacity while that derived from alphaIIb requires additional residues located adjacent to alphaIIb(995-1003). Within these two sequences there is a switch in the position of an asparaginine and leucine residue for a valine and glutamine (alphaIIb, RNRPPLEED; alphaV, RVRPPQEEQ). This switch had a dramatic effect on their inhibitory capacity and on protein-protein interactions. The two arginine and glutamic residues, juxtapositioned at identical locations in both subunits, appeared to be important in specifying the orientation by which proteins can dock to this region in alphaIIb and alphaV.

Structure of the integrin beta3 transmembrane segment in phospholipid bicelles and detergent micelles

Integrin adhesion receptors transduce bidirectional signals across the plasma membrane, with the integrin transmembrane domains acting as conduits in this process. Here, we report the first high-resolution structure of an integrin transmembrane domain. To assess the influence of the membrane model system, structure determinations of the beta3 integrin transmembrane segment and flanking sequences were carried out in both phospholipid bicelles and detergent micelles. In bicelles, a 30-residue linear alpha-helix, encompassing residues I693-H772, is adopted, of which I693-I721 appear embedded in the hydrophobic bicelle core. This relatively long transmembrane helix implies a pronounced helix tilt within a typical lipid bilayer, which facilitates the snorkeling of K716's charged side chain out of the lipid core while simultaneously immersing hydrophobic L717-I721 in the membrane. A shortening of bicelle lipid hydrocarbon tails does not lead to the transfer of L717-I721 into the aqueous phase, suggesting that the reported embedding represents the preferred beta3 state. The nature of the lipid headgroup affected only the intracellular part of the transmembrane helix, indicating that an asymmetric lipid distribution is not required for studying the beta3 transmembrane segment. In the micelle, residues L717-I721 are also embedded but deviate from linear alpha-helical conformation in contrast to I693-K716, which closely resemble the bicelle structure.

Cell adaptive response to extracellular matrix density is controlled by ICAP-1-dependent beta1-integrin affinity

Cell migration is an integrated process requiring the continuous coordinated assembly and disassembly of adhesion structures. How cells orchestrate adhesion turnover is only partially understood. We provide evidence for a novel mechanistic insight into focal adhesion (FA) dynamics by demonstrating that integrin cytoplasmic domain–associated protein 1 (ICAP-1) slows down FA assembly. Live cell imaging, which was performed in both Icap-1–deficient mouse embryonic fibroblasts and cells expressing active β₁ integrin, shows that the integrin high affinity state favored by talin is antagonistically controlled by ICAP-1. This affinity switch results in modulation in the speed of FA assembly and, consequently, of cell spreading and migration. Unexpectedly, the ICAP-1–dependent decrease in integrin affinity allows cell sensing of matrix surface density, suggesting that integrin conformational changes are important in mechanotransduction. Our results clarify the function of ICAP-1 in cell adhesion and highlight the central role it plays in the cell's integrated response to the extracellular microenvironment.

Cooperative role of the membrane-proximal and -distal residues of the integrin beta3 cytoplasmic domain in regulation of talin-mediated alpha IIb beta3 activation

Integrin cytoplasmic tails regulate integrin activation that is required for high affinity binding with ligands. The interaction of the integrin beta subunit tail with a cytoplasmic protein, talin, largely contributes to integrin activation. Here we report the cooperative interaction of the beta3 membrane-proximal and -distal residues in regulation of talin-mediated alpha IIb beta3 activation. Because a chimeric integrin, alpha IIb beta3/beta1, in which the beta3 tail was replaced with the beta1 tail was constitutively active, we searched for the residues responsible for integrin activation among the residues that differed between the beta3 and beta1 tails. Single amino acid substitutions of Ile-719 and Glu-749 in the beta3 membrane-proximal and -distal regions, respectively, with the corresponding beta1 residues or alanine rendered alphaIIbbeta3 constitutively active. The I719M/E749S double mutant had the same ligand binding activity as alpha IIb beta3/beta1. These beta3 mutations also induced alphaVbeta3 activation. Conversely, substitution of Met-719 or Ser-749 in the beta1 tail with the corresponding beta3 tail residue (M719I or S749E) inhibited alpha IIb beta3/beta1 activation, and the M719I/S749E double mutant inhibited ligand binding to a level comparable with that of the wild-type alpha IIb beta3. Knock down of talin by short hairpin RNA inhibited the I719M- and E749S-induced alpha IIb beta3 activation. These results suggest that the beta3 membrane-proximal and -distal residues cooperatively regulate talin-mediated alpha IIb beta3 activation.

SCHOOL model and new targeting strategies

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.

Modification of kidney barrier function by the urokinase receptor

Podocyte dysfunction, represented by foot process effacement and proteinuria, is often the starting point for progressive kidney disease. Therapies aimed at the cellular level of the disease are currently not available. Here we show that induction of urokinase receptor (uPAR) signaling in podocytes leads to foot process effacement and urinary protein loss via a mechanism that includes lipid-dependent activation of alphavbeta3 integrin. Mice lacking uPAR (Plaur-/-) are protected from lipopolysaccharide (LPS)-mediated proteinuria but develop disease after expression of a constitutively active beta3 integrin. Gene transfer studies reveal a prerequisite for uPAR expression in podocytes, but not in endothelial cells, for the development of LPS-mediated proteinuria. Mechanistically, uPAR is required to activate alphavbeta3 integrin in podocytes, promoting cell motility and activation of the small GTPases Cdc42 and Rac1. Blockade of alphavbeta3 integrin reduces podocyte motility in vitro and lowers proteinuria in mice. Our findings show a physiological role for uPAR signaling in the regulation of kidney permeability.

A nonsynonymous SNP in the ITGB3 gene disrupts the conserved membrane-proximal cytoplasmic salt bridge in the alphaIIbbeta3 integrin and cosegregates dominantly with abnormal proplatelet formation and macrothrombocytopenia

We report a 3-generation pedigree with 5 individuals affected with a dominantly inherited macrothrombocytopenia. All 5 carry 2 nonsynonymous mutations resulting in a D723H mutation in the beta3 integrin and a P53L mutation in glycoprotein (GP) Ibalpha. We show that GPIbalpha-L53 is phenotypically silent, being also present in 3 unaffected pedigree members and in 7 of 1639 healthy controls. The beta3-H723 causes constitutive, albeit partial, activation of the alphaIIbbeta3 complex by disruption of the highly conserved cytoplasmic salt bridge with arginine 995 in the alphaIIb integrin as evidenced by increased PAC-1 but not fibrinogen binding to the patients' resting platelets. This was confirmed in CHO alphaIIbbeta3-H723 transfectants, which also exhibited increased PAC-1 binding, increased adhesion to von Willebrand factor (VWF) in static conditions and to fibrinogen under shear stress. Crucially, we show that in the presence of fibrinogen, alphaIIbbeta3-H723, but not wild-type alphaIIbbeta3, generates a signal that leads to the formation of proplatelet-like protrusions in transfected CHO cells. Abnormal proplatelet formation was confirmed in the propositus's CD34+ stem cell-derived megakaryocytes. We conclude that the constitutive activation of the alphaIIbbeta3-H723 receptor causes abnormal proplatelet formation, leading to incorrect sizing of platelets and the thrombocytopenia observed in the pedigree.