Published online before print April 26, 2007, doi: 10.1161/ATVBAHA.107.141390
© 2007 American Heart Association, Inc.
Brief Reviews |
Platelet Receptor Proteolysis
A Mechanism for Downregulating Platelet Reactivity
From the Departments of Immunology (R.K.A., E.E.G., M.C.B.) and Biochemistry and Molecular Biology (D.K.), Monash University, Melbourne, Australia.
Correspondence to Robert K. Andrews or Michael C. Berndt, Department of Immunology, Monash University, Alfred Medical Research & Education Precinct (AMREP), Melbourne 3004, Australia. E-mail rob.andrews{at}med.monash.edu.au or michael.berndt@med.monash.edu.au
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Abstract |
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 Top Abstract IntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
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The platelet plasma membrane is literally at the cutting-edge of recent research into proteolytic regulation of the function and surface expression of platelet receptors, revealing new mechanisms for how the thrombotic propensity of platelets is controlled in health and disease. Extracellular proteolysis of receptors irreversibly inactivates receptor-mediated adhesion and signaling, as well as releasing soluble fragments into the plasma where they act as potential markers or modulators. Platelet-surface sheddases, particularly of the metalloproteinase-disintegrin (ADAM) family, can be regulated by many of the same mechanisms that control receptor function, such as calmodulin association or activation of signaling pathways. This provides layers of regulation (proteinase and receptor), and a higher order of control of cellular function. Activation of pathways leading to extracellular shedding is concomitant with activation of intracellular proteinases such as calpain, which may also irreversibly deactivate receptors. In this review, platelet receptor shedding will be discussed in terms of (1) the identity of proteinases involved in receptor proteolysis, (2) key platelet receptors regulated by proteolytic pathways, and (3) how shedding might be regulated in normal physiology or future therapeutics. In particular, a focus on proteolytic regulation of the platelet collagen receptor, glycoprotein (GP)VI, illustrates many of the key biochemical, cellular, and clinical implications of current research in this area.
This review focuses on key proteolytic mechanisms regulating the surface expression, adheso-signaling function and plasma levels of platelet receptors involved in thrombosis, inflammation and other vascular processes, in particular using platelet collagen receptor, glycoprotein (GP)VI, shedding to illustrate the implications for normal physiology or future therapeutics.
Key Words: glycoprotein Ib-IX-V • glycoprotein VI • platelets • thrombosis • shedding
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Introduction |
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TopAbstractIntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
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This review focuses on proteolytic pathways that regulate the function and surface expression of platelet receptors involved in hemostasis and thrombotic disease such as heart attack or stroke. In particular, platelet-specific receptors, glycoprotein (GP)VI and the GPIb-IX-V complex that bind collagen or von Willebrand factor (vWF), respectively, form a physical and functional complex1 initiating platelet thrombus formation at high shear stress in flowing blood.2–6 Major consequences of signal transduction following engagement of GPVI or GPIb-IX-V include (1) activation of platelet integrins, primarily
IIbß3 that binds vWF or fibrinogen and mediates platelet aggregation, and (2) activation of pathways leading to metalloproteinase-dependent receptor ectodomain shedding. Recent biochemical and cellular studies on shedding pathways7–10 and surface expression11–13 of GPVI illustrate some of the key mechanisms underlying platelet receptor proteolysis, and recent preclinical studies14 show how significant these findings may be in the context of thrombotic risk. The endogenous platelet-surface sheddases that cleave GPVI and/or other platelet receptors are also being revealed.15–17 The first part of the review, therefore, will briefly discuss the metalloproteinase-disintegrin (ADAM) family, matrix metalloproteinase (MMP) family, and the intracellular proteinase calpain, followed by a description of their potential receptor substrates on platelets. Finally, the way in which proteinases and platelet receptor expression might be regulated physiologically and/or therapeutically will be considered. Using GPVI as the main example, this review will discuss recent studies on proteolytic pathways in platelets with respect to (1) the proteinases involved, (2) key adhesion receptors, and (3) how both receptors and proteinases might be regulated. Together, the findings may be broadly applicable to other cell types, and may provide new approaches for antithrombotic therapies or assessment of thrombotic risk. |
Platelet Proteinases/Sheddases |
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TopAbstractIntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
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Members of 2 main families of proteinases are expressed on the cell surface, the ADAM family of metalloproteinase-disintegrins and the MMP family of matrix metalloproteinases, and may act as sheddases by cleaving the ectodomain of surface receptors. One of the major families of intracellular enzymes involved in regulating platelet surface receptors is the cysteinyl proteinase, calpain. There are common features in the regulation of both ectodomain sheddases and cytoplasmic domain-cleaving proteinases such as calpain. For example, both proteolytic events are inhibited by EDTA, and both may be regulated by calmodulin (below). This suggests there may be common mechanisms for switching on both extracellular and intracellular proteolytic pathways, to regulate adhesion and signaling in platelets or other cells.
ADAM Family: ADAM10 and ADAM17
ADAM10 and ADAM17 are members of the ADAM (a disintegrin and a metalloproteinase) family of cell surface proteinases,18–20 and consist of modular domains: a propeptide domain, a catalytic domain containing a conserved metal ion-coordination motif (HEXXHXXGXXH), a disintegrin domain, a Cys-rich domain, transmembrane domain, and cytoplasmic tail (Figure 1). Other members of the ADAM family contain an additional epidermal growth factor (EGF)-like domain before the transmembrane domain. Cellular, biochemical, and bioinformatic studies indicate that there are 29 forms of ADAM family metalloproteinases, and 19 forms of related ADAMTS metalloproteinases (that also include one or more thrombospondin-1 type domains; Figure 1).18–22 The main categories of ADAM function are cell adhesion, receptor shedding, and extracellular matrix degradation.21 ADAM-mediated biological roles include fertilization, reproduction, neuromuscular development, angiogenesis, inflammation, cancer, and as intimated below, thrombosis.18–21 On platelets, ADAM10 is involved in the proteolytic processing of amyloid precursor protein (APP), and expression levels may be a marker for Alzheimer disease.23 ADAM family metalloproteinases contain a signal peptide, and a propeptide domain, the removal of which after furin-mediated or other processing produces an active form of the enzyme. The prodomain of ADAM10 and ADAM17 contains a cysteine switch, that is, a free sulfydryl involved in maintaining the metalloproteinase in an inactive form, with activation of the enzyme occurring after proteolytic removal of the prodomain or on modification by thiol-modifying reagents such as N-ethylmaleimide (NEM; refer below).20,24–27 As shown in Figure 1, mammalian ADAM family metalloproteinases are structurally related to snake venom metalloproteinase-disintegrins which in their mature form consist of a catalytic domain, a disintegrin domain, and a Cys-rich domain.28 Vascular apoptosis factor-1 (VAP-1), a rattlesnake venom metalloproteinase-disintegrin from Crotalus atrox, was recently used to obtain the first crystal structure for an intact ADAM (metalloproteinase domain, disintegrin domain, and Cys-rich domain), revealing how these domains are configured. VAP-1 forms a C-shaped structure, with the metalloproteinase and Cys-rich domains forming globular regions at either end of the ‘C’, with the intervening disintegrin linking these two regions.29 The Cys-rich domain is situated near the catalytic site, suggesting it acts as a regulatory domain.29 Previous studies involving mammalian ADAM10 show that a recombinant disintegrin/Cys-rich (dc) fragment regulates ADAM10-dependent proteolysis of the ephrin receptor complex.30
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ADAM10 and/or ADAM17 regulate ectodomain shedding of receptors on vascular cells including endothelial cells, leukocytes or platelets, and other cell types.18–20 A lot is known about the functional importance of these sheddases in mammalian biology, but relatively little is known about the detailed mechanisms for their mode of action and regulation. As discussed in detail elsewhere,19 selective inhibitors are currently not widely available and there is minimal conservation within cleavage sites. Both ADAM10 and ADAM17 are expressed on resting platelets,15–17,23 and one or both have been directly linked to shedding of platelet receptors (below). In this regard, two common properties of ADAM10 and ADAM17 are relevant to their role as platelet sheddases. First, as originally shown by Kahn et al for leukocyte L-selectin,31 ADAM-mediated shedding is regulated by the cytosolic regulatory protein, calmodulin. Calmodulin binds to a juxtamembrane, positively-charged/hydrophobic sequence within the cytoplasmic tail of L-selectin, analogous to calmodulin-binding sequences that form an amphipathic -helix in other proteins.32 Disruption of this interaction by mutagenesis of the calmodulin-binding site of the receptor, or by treating L-selectin–expressing cells with calmodulin inhibitors, induces ectodomain shedding.31 This implies calmodulin associated with the receptor acts as an inhibitor of shedding. Further evidence shows that calmodulin also regulates ADAM function. For example, calmodulin regulates proteolytic processing and activation of ADAM10, because the cytoplasmic domain of pro-ADAM10 interacts directly with calmodulin in human glioma cells, and calmodulin inhibitors increase conversion of pro-ADAM10 to active ADAM10 (and promote ADAM10-mediated proteolysis of CD44).33 Second, activation of intracellular signaling pathways may be involved in inside-out activation of surface sheddases.18–20 The cytoplasmic tails of ADAM10 and/or ADAM17 contain motifs that interact with signaling molecules, including consensus sequences for binding Src homology (SH)-2 and SH-3 domains, even though precise pathways are far from fully understood. As discussed further below, ADAM-dependent ectodomain shedding may be induced by the phorbol ester, PMA, which activates protein kinase C (PKC),16–20 or alternately, blocked by inhibitors of cell signaling pathways.8 On other cells, ADAM10 or ADAM17 may be differentially regulated, by Ca2+/calmodulin or PKC-dependent pathways,31 and related mechanisms may control shedding of platelet receptors.
MMP
Members of the MMP family exist as secreted or surface-expressed forms, which include a transmembrane domain or are GPI-linked.34 They contain a propeptide (inhibitory Cys-switch) domain, a catalytic, and other domains. Four members of the MMP family, MMP-1, MMP-2, MMP-4, and MMP-9 have been identified on platelets,35–38 and can play a role in platelet aggregation. MMP-2, and the colocalized metalloproteinase inhibitor, tissue inhibitor of metalloproteinase-4 (TIMP-4), are released from collagen- or thrombin-stimulated platelets; the relative concentrations of MMP-2 and TIMP-4 regulate MMP-2-dependent platelet aggregation.39 A role for MMP family metalloproteinases in regulating platelet receptor ectodomain shedding has been suggested9 but not yet definitively shown.
Calpain
Calpain is a ubiquitous intracellular cysteinyl proteinase, switched on in activated platelets and regulated, in part, by intracellular Ca2+ levels.40,41 The 3 isoforms of calpain, acting on >100 intracellular substrates,41,42 play a role in regulating cytoskeletal rearrangements associated with cell motility, adhesion, and division. Notably in the context of activation of sheddases discussed here, the ability of a protein to bind calmodulin confers a strong likelihood that this protein is a substrate for calpain.43 In other words, mechanisms for activating extracellular shedding pathways of proteolysis also have the potential to activate intracellular calpain-dependent proteolytic pathways via a calmodulin-dependent mechanism. Interestingly, EDTA inhibits both metalloproteinase-mediated receptor shedding and calpain activity,44 presumably, in the latter case, by chelating extracellular calcium and interfering with Ca2+ flux. In platelets, the most prevalent µ-calpain isoform is localized to focal adhesions, where it regulates shape change, motility, and adhesion mainly through modulation of integrin clustering and function.45–49 Calpain also regulates IIbß3 on activated platelets by cleavage of the ß3 cytoplasmic tail, resulting in the removal of 2 NXXY motifs that are important for ligand binding, bidirectional signaling, and cytoskeletal attachment.50–52 Four calpain-dependent cleavage sites flanking the NXXY motifs have been identified.50 One of the critical functions of the cytoplasmic domain of IIbß3 is the regulation of clot contraction, involving association of the receptor with contractile actin filaments of the cytoskeleton in activated platelets. Proteolysis of the cytoplasmic tail of ß3 by calpain abolishes IIbß3-dependent clot contraction. Both calmodulin and calpain have been implicated in regulating the intracellular proteolysis of the cytoplasmic domain of platelet-endothelial cell adhesion molecule-1 (PECAM-1) in platelets,53,54 supporting the possibility of dual extracellular (sheddase) and intracellular (calpain) proteolytic pathways operating in tandem in platelets (below).
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Platelet Receptors |
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TopAbstractIntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
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GPVI
Glycoprotein (GP)VI (
65 kDa) is a member of the immunoglobulin (Ig) superfamily, with two extracellular Ig domains, a mucin-like domain, transmembrane domain, and cytoplasmic tail.5,6 In addition to the major physiological ligand, collagen, GPVI also binds laminin55 and nonphysiological ligands including cross-linked collagen-related peptide (CRP) and the snake toxins, convulxin, and alborhagin, which bind at overlapping but distinct binding-sites within the extracellular Ig domains.56–58 GPVI signals by Fc receptor 
-chain (FcR)-dependent and FcR-independent pathways5,6,59–61: the former involves an immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic tail of FcR, whereby ligand-induced cross-linking of GPVI/FcR leads to activation of the ITAM-dependent Syk kinase; this involves direct interaction of Src family kinases, Fyn and Lyn, with a consensus Pro-rich sequence in the GPVI cytoplasmic tail,62 whereas a juxtamembrane calmodulin-binding sequence is involved in GPVI-dependent Ca2+ signaling.61,62 These signaling pathways result in secretion of agonists such as ADP, and inside-out activation of platelet integrins, primarily IIbß3 that binds fibrinogen or vWF and mediates platelet aggregation.
Another major consequence of GPVI signaling induced by the GPVI ligands, collagen, CRP, and convulxin, is the activation of proteolytic pathways leading to GPVI ectodomain shedding,7–10 and the release of a soluble 55-kDa fragment into the platelet supernatant, with an 10-kDa remnant fragment that remains platelet-associated. GPVI shedding is metalloproteinase-dependent, and inhibitable by EDTA, by a broad-specificity metalloproteinase inhibitor GM6001, and by tumor necrosis factor (TNF)- protease inhibitor (TAPI).9 Ligand-induced shedding also requires GPVI signaling, and is blocked by inhibitors of GPVI-related signaling proteins, Src family kinases (PP1 or PP2), PI3-kinase (wortmannin), and Syk kinase (piceatannol).8 Signaling also leads to rapid dissociation of calmodulin from the cytoplasmic tail, and calmodulin inhibitors such as W7 that dissociate calmodulin, also induce GPVI ectodomain shedding.8,10,63 Alternately, GPVI shedding is induced by NEM,1 which can directly activate sheddases.20,21 Importantly, W7- or NEM-induced shedding1,8 is independent of platelet activation, and is unaffected by blocking Src (PP1 or PP2) or Syk (piceatannol) (unpublished result, 2007). Recent evidence suggests that the sheddase involved in GPVI proteolysis is ADAM10.64 This implies, therefore, that shedding of GPVI and GPIb (see below) may involve distinct mechanisms, and that potentially, their expression may be differentially regulated. In this regard, in a platelet fully activated by a GPVI agonist, all of the detectable GPVI is shed while GPIb and other platelet receptors remain almost fully expressed on the cell surface.8 The time-frame for shedding of GPVI from human platelets in vitro is minutes to hours, whether induced by NEM, W7, or GPVI ligands1,8,9; in general, more potent agonists such as convulxin induce more rapid shedding than CRP or collagen, consistent with signaling-dependent activation of metalloproteinases. In vivo, it is possible that shedding may occur within minutes, particularly under shear conditions or on platelet aggregation (particularly if, like ADAM10-mediated shedding of the ephrin receptor complex,30 GPVI shedding occurs in trans; Figure 2A). It is notable, however, that although shedding of platelet CD40 ligand is regulated by IIbß3-dependent signaling (see below), GPVI shedding is unaffected by IIbß3 blockade.8 There is increasing evidence that expression levels of GPVI regulate in a graded fashion platelet reactivity toward collagen and other ligands.11–13 There is a correlation between GPVI levels on human or murine platelets and adhesion to collagen under flow conditions. In clinical studies,14 the levels of human platelet GPVI correlate with acute coronary syndrome, suggesting that elevated levels of platelet GPVI may act as a marker of impending acute coronary events.
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GPIb-IX-V
Platelet GPIb-IX-V is a complex of GPIb disulfide-linked to GPIbß and forming a noncovalent complex with GPIX and GPV.2,3 GPIb is the major ligand-binding subunit of the receptor complex, and consists of a globular 45-kDa N-terminal ligand-binding domain, a sialomucin core, a transmembrane domain, and a cytoplasmic tail. Immediately N-terminal to the transmembrane domain is a pair of Cys residues, one or both of which form a disulfide-bond with GPIbß.65 The ligand-binding domain (His1-Glu282) consists of 7 leucine-rich repeats, their N- and C-terminal flanking sequences, and an anionic sequence with sulfation at Tyr276, Tyr278, and Tyr279.2,3 Glycocalicin, a soluble ectodomain fragment of GPIb (N-terminal and sialomucin domains), is present in plasma at 1 to 3 µg/mL,66 and is formed by cleavage N-terminal to the disulfide bond linking GPIb to GPIbß. GPIb shedding occurs in aging platelets, or platelets treated with the mitochondrial-targeting reagent, CCCP (carbonyl cyanide 3-chlorophenylhydrazone), and is mediated by ADAM17, because purified recombinant ADAM17 cleaves glycocalicin from platelets, and dysfunctional ADAM17 in genetically-modified mice leads to decreased plasma glycocalicin and decreased ectodomain shedding.16,17 Like GPVI,63 the cytoplasmic domain of GPIb-IX-V directly binds calmodulin at juxtamembrane sequences of GPIbß and GPV.67 In the case of GPV, calmodulin inhibitors also induce ectodomain shedding, by a mechanism that, like GPIb, involves activation of ADAM17.15
Shedding of GPIb is likely to have multiple effects on platelet function. There are multiple ligands for GPIb2 (Figure 2B) that regulate platelet adhesion to subendothelial matrix (vWF or thrombospondin68), activated endothelial cells (P-selectin or vWF/P-selectin),69–71 or leukocytes (Mß2).72 Recent studies show that whereas thrombus formation is markedly impaired in transgenic mice where platelets express a GPIb-IX-V complex lacking the GPIb ligand-binding ectodomain, vWF-deficient mice are similar to control mice72,74; the authors suggest GPIb binding to ligand(s) other than vWF is critical in these models. GPIb is known to play a key role in regulating coagulation at the surface of activated platelets, by binding to coagulation factors XI75,76 and XII,77 high molecular weight kininogen,78 and thrombin.79–83 The interaction of thrombin with GPIb (1) facilitates thrombin-dependent activation of PAR-1,83,84 (2) promotes activation of GPIb-associated factor XI (potentially regulated by factor XII),75–77 and (3) enables direct signaling via GPIb in the absence of GPV.85 Shedding of GPV,15 therefore, is likely to control thrombin-dependent platelet activation via GPIb; GPV also supports platelet adhesion to collagen.86
GPIb-IX-V and GPVI are physically and functionally coassociated on the platelet plasma membrane,1 and are thought to act synergistically as mutual accessory signaling receptors, particularly at low agonist concentration.87 The ectodomains of GPIb (glycocalicin) and GPVI (55-kDa fragment) directly interact,1 and this could account for their functional coassociation. Therefore, shedding of either receptor is likely to adversely affect activation thresholds of the companion receptor. Both GPVI (FcR)60,61 and GPIb-IX-V (FcRIIa)88 indirectly use ITAM-bearing receptors, such that decreased ligand binding to either receptor as a result of shedding would attenuate ITAM-dependent signaling in platelets. In other pathology where receptor expression levels may be relevant, GPIb binds ß2-glycoprotein I, and GPIb-mediated platelet activation by autoimmune complexes involving this ligand provides a mechanism for thrombus associated with antiphospholipid syndrome.89
PECAM-1
Like GPVI, PECAM-1 is a member of the Ig superfamily, with 6 extracellular Ig domains, transmembrane domain, and cytoplasmic tail. Unlike GPVI, the cytoplasmic domain of PECAM-1 contains an immunoreceptor tyrosine-based inhibitory motif (ITIM). PECAM-1 plays a role in attenuating thrombus formation involving GPVI or other receptors90–92; in this regard, PECAM-1 is physically and functionally associated with FcRIIa on human platelets, and also negatively regulates collagen-dependent stimulation of GPVI/FcR.93 Interestingly, like GPVI, PECAM-1 contains a calmodulin-binding sequence in the juxtamembrane region of the cytoplasmic tail, and calmodulin inhibitors induce proteolysis of PECAM-1.53 Other studies show that the cytoplasmic domain of PECAM-1 is cleaved by calpain in activated platelets at a site upstream of the ITIM, thereby deactivating the receptor.54 This suggests that the intracellular proteinase, calpain, is activated under the same conditions that activate ectodomain metalloproteinases in platelets (in response to calmodulin inhibitors), implying both extracellular and intracellular sites of receptors are targeted when proteolytic pathways are switched on.
Sema4D
One recent article shows that human platelets express the immune cell receptor, semaphorin 4D (Sema4D), and its binding partners, CD72 and plexin-B1.94 Sema4D, CD72 (associated with the tyrosine phosphatase, SHP-1) and plexin-B1 contribute to the regulation of thrombus formation, and mice deficient in Sema4D show decreased occlusive thrombi in arterial thrombosis models.94 Surface expression of these proteins increases on platelets activated with PMA, collagen, or other agonists, and metalloproteinase-mediated shedding of Sema4D releases a soluble ectodomain fragment. Shedding of Sema4D is inhibited in ADAM17-defective mice, supporting a role for ADAM17 in cleavage of platelet receptors following activation.
CD40L
Platelet CD40 ligand regulates stability of IIbß3-dependent thrombi.95,96 It is shed from activated platelets by an IIbß3-dependent mechanism (shedding is blocked by inhibition of ligand binding to IIbß3).96 This is in contrast to GPVI shedding, which is independent of IIbß3.8 The physiological relationship between shedding of CD40L and other adhesion receptors and the effects of this on thrombus formation in vivo are not yet resolved.
G Protein–Coupled Receptors: PAR-1
G protein–coupled receptors may also be regulated by extracellular proteolytic pathways. Precedence for this is the metalloproteinase-mediated cleavage of the ligand-binding extracellular N-terminal domain of the thrombin protease-activated receptor-1 (PAR-1) from the surface of human endothelial cells.97 ADAM17 (or a related proteinase) is implicated in mediating PAR-1 cleavage, because it is inhibited by TNF- protease inhibitor-2 (TAPI-2) and tissue inhibitor of metalloproteinase-3 (TIMP-3), but not TIMP-1 or -2. A specific role for calmodulin in this process is not established, but other G protein–coupled receptors including the platelet ADP receptor, P2Y1,98 directly bind calmodulin. The generality or physiological role of G protein–coupled receptor shedding on platelets is currently uncertain, but presumably occurs at the same time as adhesion receptor shedding involving ADAM17 or other sheddases.
Platelet Integrin Shedding
The function of both platelet and leukocyte integrins is directly relevant to the function of platelet GPIb-IX-V and GPVI. Primarily, the platelet-specific integrin, IIbß3 (GPIIb-IIIa), plays a key role in thrombus formation, by binding vWF, fibrinogen, or other ligands to mediate platelet aggregation, after ligand binding to GPIb-IX-V, GPVI, or other receptors.2 Platelets also express the vitronectin receptor, vß3, and ß1 integrins 1ß1, 2ß1, 5ß1, and 6ß1, which bind adhesive ligands including collagen, laminin, and/or fibronectin.99 The leukocyte integrin, Mß2 (Mac-1), is involved in leukocyte adhesion to the vessel wall, and is activated after initial contact of leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) with P-selectin expressed on activated endothelial cells or activated mural platelets. This enables Mß2-mediated firm adhesion either to the endothelial immunoglobulin family receptor, intercellular adhesion molecule-1 (ICAM-1), or to platelet GPIb-IX-V.73 The inserted (or I) domain of M mediates binding of Mß2 to GPIb, ICAM-1 or other ligands.
Whereas the proteolytic regulation of ß3 integrins by calpain has been reported (refer above),50–52 the possibility that ectodomain shedding plays a role in regulating the function of platelet integrins has not been widely explored. There is, however, precedence for integrin shedding on other cells. For example, recent studies involving another leukocyte ß2 integrin, Lß2, show that an as yet unidentified cell surface proteinase(s) cleaves both a membrane-proximal region of ß2 and a more upstream site of L, releasing a soluble ectodomain fragment of L (containing the I domain) in complex with the ectodomain of ß2, from the surface of blister-fluid neutrophils.100,101 Cell surface-expressed ADAM family receptors are known to interact via their distintegrin domains with integrins on other cells. For example, ADAM15, principally via its RGD-containing disintegrin domain, acts as a counter-receptor for IIbß3.102 It is tempting to speculate a broader role for ADAM family metalloproteinases in platelet integrin shedding, as suggested for shedding of platelet GPIb-IX-V, GPVI, and other receptors.
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Regulation of Proteinases/Receptors |
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Calmodulin
Despite evidence that calmodulin binds directly to juxtamembrane cytoplasmic sequences of sheddase substrates, including GPV, GPVI, L-selectin, and other receptors, and that dissociation facilitates ectodomain shedding,8,15,31,61,63 the mechanism for precisely how this occurs is obscure. Calmodulin might maintain the receptor in a protected, noncleavable state on resting cells by conformational changes across the membrane affecting ectodomain cleavage sites. Calmodulin-binding sequences consisting of positively-charged/hydrophobic amphipathic helix in the cytoplasmic juxtamembrane position may be involved in stable membrane insertion. In this case, disruption of calmodulin by mutagenesis or by treatment with calmodulin inhibitors such as W7 or trifluoperazine, might enable increased exposure of extracellular juxtamembrane sheddase-cleavage sites above the lipid bilayer facilitating hydrolysis. Calmodulin-binding sequences occur on 3 key platelet receptors that initiate thrombosis at arterial shear rates,74,103–105 GPIb-IX-V, GPVI, and P2Y1, and may play additional roles other than regulating ectodomain shedding. The calmodulin-binding sequence of GPVI, for example, is critical for initiating ligand-induced Ca2+ signaling in GPVI-transfected cell lines.61 Similarly a major functional consequence of ADP binding to G
q-linked P2Y1 is elevation of cytosolic Ca2+ upstream of signaling by the other platelet ADP receptor, Gi-linked P2Y12.98,103 Analogous calmodulin-binding sequences in epidermal growth factor receptor (EGFR), MARCKS, and other proteins interact strongly with the plasma membrane and sequester phosphatidylinositol bisphosphate (PIP2), a multifunctional mediator, which is also upstream of inositol triphosphate (IP3)-dependent release of Ca2+ from internal stores.106 Finally, calmodulin inhibitors may also act, in part, by activation of ADAM10 and/or ADAM17, and there is some evidence for a direct interaction between calmodulin and pro-ADAM10, which may regulate proteolytic processing.18–20,33 Nevertheless, in platelets, calmodulin inhibitors induce shedding of GPIb, GPV, and GPVI (without the requirement for platelet activation or signaling), suggesting that calmodulin-receptor interaction could be therapeutically targeted to decrease functional receptor expression.10
NEM
As mentioned earlier, ADAM10 and ADAM17 contain a Cys switch in the prodomain, allowing these proteinases to be directly activated by thiol-modifying reagents such as NEM.20,24 MMP sheddases may also contain a regulatory Cys switch.34 Treating human platelets with NEM has been shown to induce shedding of the 55-kDa ectodomain fragment of GPVI,1 in the absence of GPVI ligands or platelet activation. Like calmodulin inhibitors, therefore, NEM treatment provides a mechanism for directly inducing loss of platelet surface GPVI (or other receptors shed by ADAM10 or ADAM17) without accompanying platelet activation. Although using thiol-modifying agents such as NEM in vivo would presumably be limited, this supports the concept that direct activation of sheddases could be exploited therapeutically.
Aging
Platelet receptor expression levels have also been investigated with respect to platelet aging and clearance; anucleate platelets circulate in the bloodstream for 1 week. In dogs, platelet responsiveness to collagen decreases with age, correlating with an age-dependent decrease in GPVI-dependent platelet activation.107 In zebra fish, an age-related decrease in adhesion receptor levels on nucleated thrombocytes also correlates with decreased participation in thrombus formation; younger thrombocytes are more adhesive, and have a higher propensity to form thrombi.108 A specific role for platelet sheddases in regulating age-related changes in adhesion receptor levels has not been directly demonstrated. However, as discussed above, mimicking aging, by treating platelets with the mitochondrial poison CCCP, induces ADAM17-mediated GPIb shedding.16 The key question of whether shedding is a determinant or a consequence of platelet aging has also not been addressed.
Nonphysiological Ligands
Another mechanism for inducing shedding of receptors is binding of ligands. Nieswandt and colleagues originally showed that the anti-mouse GPVI monoclonal antibody, JAQ1, also selectively decreased GPVI surface expression,109 involving, at least in part, metalloproteinase-mediated shedding. Similarly, human platelets in a NOD/SCID mouse model can be depleted of GPVI in an activation-independent manner by anti-human GPVI antibodies.110 Whether proteolysis of GPVI in this study was the result of ADAM-mediated shedding was not directly assessed, however Western blots with antibodies against the GPVI cytoplasmic tail indicate that the soluble plasma form of human GPVI generated in this model lacks the cytoplasmic domain.110 Anti-GPVI antibody treatment provides a mechanism for selectively down-regulating platelet GPVI surface expression in vivo (GPVI is essentially platelet-specific).
PMA
The phorbol ester, phorbol 12-myristate 13-acetate (PMA) activates platelet protein kinase C (PKC), and also induces metalloproteinase-mediated ectodomain shedding of GPIb and GPV.16,17 Although treatment with PMA stimulates platelet activation, it is possible that dissecting this pathway leading to activation of ADAM sheddases could identify targets allowing activation-independent downregulation of platelet adhesion receptors.
Aspirin
One recent report shows that aspirin promotes ADAM17-mediated shedding of GPIb and GPV from human or mouse platelets, with increased levels of the respective ectodomain fragments in plasma.17 The classical target for aspirin as an antithrombotic, cyclooxygenase-1 (COX-1), was unlikely to be responsible for aspirin-related metalloproteinase-mediated shedding; however, because shedding appeared normal in COX-1–deficient mice,17 the mechanism may instead involve acylation of ADAM17 and/or substrate(s).
Diagnostics/Exposure of Cryptic Sites by Proteinases
There are at least 3 areas where an increased understanding of platelet receptor shedding mechanisms might be applied diagnostically. First, although platelet levels of GPVI have been correlated to risk of acute coronary syndrome,14 little evidence exists on whether plasma GPVI levels are indicative of thrombotic risk. One study shows that GPVI is present in normal human plasma,111 but further developments are needed to facilitate analytical screening of patient samples. Second, although platelet levels of intact GPVI have been assessed by flow cytometry, the 10-kDa remnant fragment of GPVI on platelets has not been directly measured. Antibodies against the GPVI cytoplasmic domain have been used to analyze this fragment by Western blot,109 however antibodies against the new N terminus of cleaved GPVI could be used in flow cytometry to determine ratios of intact versus shed receptor as an index of thrombotic exposure. The exploitation of cryptic postproteolytic sites and fragments has recently been reviewed elsewhere.112 Third, proteinase activity assays, for example, based on consensus substrates developed for ADAM10 and ADAM17,113 could be used to assess sheddase activity levels on resting versus stimulated platelets as an additional measure of surface expression status of GPVI (or other platelet receptors). Total ADAM10 or ADAM17 levels are measurable by flow cytometry,23 and determining specific activity of surface metalloproteinase could indirectly comment on thrombotic potential. Given the clinical importance of GPVI levels,14 development of these and other methods is warranted.
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Clinical Implications of Platelet Proteolysis |
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TopAbstractIntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
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The pathology of thrombus formation in thrombotic disease can be described as a series of steps. Atherosclerotic plaque rupture or vascular injury exposes the subendothelial matrix, supporting platelet translocation, then rapid adhesion via exposed vWF and collagen, initiated by binding to platelet GPIb-IX-V and GPVI/FcR
, respectively.1–6,74,105,114,115 Adhesive interactions involving these receptors enables signal transduction and platelet activation, with secretion of agonists such as ADP, and activation of platelet integrins, in particular IIbß3 that binds vWF or fibrinogen, resulting in firm adhesion, spreading, and recruitment of additional platelets from the bloodstream into the developing thrombus.73,116–118 Arterial thrombus formation in vivo is a dynamic process, with developing thrombus at the site of injury gradually building up over time but not without large fragments becoming detached from the thrombus mass, and subsequent rebuilding by additional incoming platelets.118,119–121 That is, after initiation, the degree of thrombus stability is a determinant of the overall rate and progression of thrombus build-up, before either entirely occluding the blood vessel, or reaching a state of surface passification where thrombus growth is limited and thrombus size is stabilized.121 Ectodomain shedding of platelet adheso-signaling receptors, GPIb and GPVI, would provide an attractive mechanism for controlling thrombus growth and stability. Several lines of evidence support this contention. First, platelet spreading on prothrombotic surfaces bearing vWF and/or collagen would require the detachment (then possibly reattachment) at the tips of filopodia or the leading edge of lamellipodia. Receptor shedding would facilitate this process. Second, shedding of GPIb could limit vWF at the surface of the developing thrombus and limit platelet translocation. Receptor shedding would thus enhance passification of the thrombus surface. Third, because both GPIb- and GPVI-dependent signaling involve receptor cross-linking, shedding of GPIb and GPVI could lower surface density of intact receptor and thereby attenuate platelet signaling, activation, and secretion. Ectodomain shedding of GPIb and GPVI would also conceivably facilitate thrombus embolization by decreasing the number of platelet-matrix contacts. Future studies should evaluate these possibilities in vivo, as the underlying molecular mechanisms are determined.
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Acknowledgments |
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Disclosures
None.
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Original received January 31, 2007; final version accepted April 16, 2007.
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References |
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TopAbstractIntroductionPlatelet Proteinases/SheddasesPlatelet ReceptorsRegulation of...Clinical Implications of...References |
|---|
1. Arthur JF, Matzaris M, Gardiner EE, Taylor SG, Wijeyewickrema LC, Ozaki Y, Kahn ML, Andrews RK, Berndt MC. Glycoprotein (GP)VI is associated with GPIb-IX-V on the membrane of resting and activated platelets. Thromb Haemostas. 2005; 93: 716–723.[Medline] [Order article via Infotrieve]
2. Andrews RK, Berndt MC, López JA. The glycoprotein Ib-IX-V complex. In: Platelets. (2nd edition) A.Michelson (ed), San Diego: Academic Press, 2006; 145–163.
3. Andrews RK, Gardiner EE, Shen Y, Whisstock JC, Berndt MC. Glycoprotein Ib-IX-V. Int J Biochem Cell Biol. 2003; 35: 1170–1174.[CrossRef][Medline] [Order article via Infotrieve]
4. Kroll MH, Hellums JD, McIntire LV, Schafer AI, Moake JL. Platelets and shear stress. Blood. 1996; 88: 1525–1541.
5. Nieswandt B, Watson SP. Platelet-collagen interaction: is GPVI the central receptor. Blood. 2003; 102: 449–461.
6. Moroi M, Jung SM. Platelet glycoprotein VI: its structure and function. Thromb Res. 2004; 114: 221–233.[CrossRef][Medline] [Order article via Infotrieve]
7. Bergmeier W, Rabie T, Strehl A, Piffath C, Prostredna M, Wagner D, Nieswandt B. GPVI down-regulation in murine platelets through metalloproteinase-dependent shedding. Thromb Haemost. 2004; 91: 951–958.[Medline] [Order article via Infotrieve]
8. Gardiner EE, Arthur JF, Kahn ML, Berndt MC, Andrews RK. Regulation of platelet membrane levels of glycoprotein VI by a platelet-derived metalloproteinase. Blood. 2004; 104: 3611–3617.
9. Stephens G, Yan Y, Jandrot-Perrus M, Villeval JL, Clemetson KJ, Phillips DR. Platelet activation induces metalloproteinase-dependent GPVI cleavage to down-regulate platelet reactivity to collagen. Blood. 2005; 105: 186–191.
10. Gardiner EE, Arthur JF, Berndt MC, Andrews RK. Role of calmodulin in platelet receptor function. Curr Med Chem Cardiovasc Hematol Agents. 2005; 3: 283–287.[CrossRef][Medline] [Order article via Infotrieve]
11. Chen H, Locke D, Liu Y, Liu C, Kahn ML. The platelet receptor GPVI mediates both adhesion and signalling responses to collagen in a receptor density-dependent fashion. J Biol Chem. 2002; 277: 3011–3019.
12. Best D, Senis YA, Jarvis GE, Eagleton HJ, Roberts DJ, Saito T, Jung SM, Moroi M, Harrison P, Green FR, Watson SP. GPVI levels in platelets: relationship to platelet function at high shear. Blood. 2003; 102: 2811–2818.
13. Sarratt KL, Chen H, Zutter MM, Santoro SA, Hammer DA, Kahn ML. GPVI and 2ß1 play independent critical roles during platelet adhesion and aggregate formation to collagen under flow. Blood. 2005; 106: 1268–1277.
14. Bigalke B, Lindemann S, Ehlers R, Seizer P, Daub K, Langer H, Schonberger T, Kremmer E, Siegel-Axel D, May AE, Gawaz M. Expression of platelet collagen receptor glycoprotein VI is associated with acute coronary syndrome. Eur Heart J. 2006; 27: 2165–2169.
15. Rabie T, Strehl A, Ludwig A, Nieswandt B. Evidence for a role of ADAM17 (TACE) in the regulation of platelet glycoprotein V. J Biol Chem. 2005; 280: 14462–14468.
16. Bergmeier W, Piffath CL, Cheng G, Dole VS, Zhang Y, von Andrian UH, Wagner DD. Tumor necrosis factor--converting enzyme (ADAM17) mediates GPIb shedding from platelets in vitro and in vivo. Circ Res. 2004; 95: 677–683.
17. Aktas B, Pozgajova M, Bergmeier W, Sunnarborg S, Offermanns S, Lee D, Wagner DD, Nieswandt B Aspirin induces platelet receptor shedding via ADAM17 (TACE). J Biol Chem. 2005; 280: 39716–39722.
18. Blobel CP. ADAMs: key components in EGFR signalling and development. Nat Rev Mol Cell Biol. 2005; 6: 32–43.[CrossRef][Medline] [Order article via Infotrieve]
19. Seals DF, Courtneidge SA. The ADAMs family of metalloproteases: multidomain proteins with multiple functions. Genes Dev. 2003; 17: 7–30.
20. Huovila APJ, Turner AJ, Pelto-Huikko M, Karkkainen I, Ortiz RM. Shedding light on ADAM metalloproteinases. Trends Biochem Sci. 2005; 30: 413–422.[CrossRef][Medline] [Order article via Infotrieve]
21. Roy R, Zhang B, Moses MA. Making the cut: protease-mediated regulation of angiogenesis. Exp Cell Res. 2006; 312: 608–622.[CrossRef][Medline] [Order article via Infotrieve]
22. Nicholson AC, Malik S-B, Logsdon Jr JM, Van Meir EG Functional evolution of ADAMTS genes: evidence from analyses of phylogeny and gene organization. BMC Evol Biol. 2005; 511–23.
23. Colciaghi F, Marcello E, Borroni B, Zimmermann M, Caltagirone C, Cattabeni F, Padovani A, Di Luca M. Platelet APP, ADAM 10 and BACE alterations in the early stages of Alzheimer disease. Neurology. 2004; 62: 498–501.
24. Amit T, Amit T, Hochberg Z, Yogev-Falach M, Youdim MB, Youdim MB, Barkey RJ. Shedding of growth hormone-binding protein is inhibited by hydroxamic acid-based protease inhibitors: proposed mechanism of activation of growth hormone-binding protein secretase. J Endocrinol. 2001; 169: 397–407.[Abstract]
25. Srour N, Lebel A, McMahon S, Fournier I, Fugere M, Day R, Dubois CM. TACE/ADAM-17 maturation and activation of sheddase activity require proprotein convertase activity. FEBS Lett. 2003; 554: 275–283.[CrossRef][Medline] [Order article via Infotrieve]
26. Endres K, Anders A, Kojro E, Gilbert S, Fahrenholz F, Postina R. Tumor necrosis factor- converting enzyme is processed by proprotein-convertases to its mature form which is degraded upon phorbol ester stimulation. Eur J Biochem. 2003; 270: 2386–2393.[Medline]
[Order article via Infotrieve]
27. Leonard JD, Lin F, Milla ME. Chaperone-like properties of the prodomain of TNF-converting enzyme (TACE) and the functional role of its cysteine switch. Biochem J. 2005; 387: 797–805.[CrossRef][Medline]
[Order article via Infotrieve]
28. Wijeyewickrema LC, Berndt MC, Andrews RK. Snake venom probes of platelet adhesion receptors and their ligands. Toxicon. 2005; 45: 1051–1061.[Medline] [Order article via Infotrieve]
29. Takeda S, Igarashi T, Mori H, Araki S. Crystal structures of VAP1 reveal ADAMs’ MDC domain architecture and its unique C-shaped scaffold. EMBO J. 2006; 25: 2388–2396.[CrossRef][Medline] [Order article via Infotrieve]
30. Janes PW, Sah N, Barton WA, Kolev MV, Wimmer-Kleikamp SH, Nievergall E, Blobel CP, Himanen JP, Lackmann M, Nikolov DB. Adam meets Eph: an ADAM substrate recognition module acts as a molecular switch for ephrin cleavage in trans. Cell. 2005; 123: 291–304.[CrossRef][Medline] [Order article via Infotrieve]
31. Kahn J, Walcheck B, Migaki GI, Jutila MA, Kishimoto TK. Calmodulin regulates L-selectin adhesion molecule expression and function through a protease-dependent mechanism. Cell. 1998; 92: 809–818.[CrossRef][Medline] [Order article via Infotrieve]
32. Hoeflich KP, Ikura M. Calmodulin in Action: Diversity in Target Recognition and Activation Mechanisms. Cell. 2002; 108: 739–742.[CrossRef][Medline] [Order article via Infotrieve]
33. Nagano O, Murakami D, Hartmann D, De Strooper B, Saftig P, Iwatsubo T, Nakajima M, Shinohara M, Saya H. Cell-matrix interaction via CD44 is independently regulated by different metalloproteinases activated in response to extracellular Ca2+ influx and PKC activation. J Cell Biol. 2004; 165: 893–902.
34. Visse R, Nagase H. Matrix metalloproteinases and tissue inhibitors of metalloproteinases: structure, function, and biochemistry. Circ Res. 2003; 92: 827–839.
35. Galt SW, Lindemann S, Allen L, Medd DJ, Falk JM, McIntyre TM, Prescott SM, Kraiss LW, Zimmerman GA, Weyrich AS. Outside-in signals delivered by matrix metalloproteinase-1 regulate platelet function. Circ Res. 2002; 90: 1093–1099.
36. Sawicki G, Salas E, Murat J, Miszta-Lane H, Radomski MW. Release of gelatinase A during platelet activation mediates aggregation. Nature. 1997; 386: 616–619.[CrossRef][Medline] [Order article via Infotrieve]
37. Fernandez-Patron C, Martinez-Cuesta MA, Salas E, Sawicki G, Wozniak M, Radomski MW, Davidge ST. Differential regulation of platelet aggregation by matrix metalloproteinases-9 and -2. Thromb Haemost. 1999; 82: 1730–1735.[Medline] [Order article via Infotrieve]
38. Sawicki G, Sanders EJ, Salas E, Wozniak M, Rodrigo J, Radomski MW. Localization and translocation of MMP-2 during aggregation of human platelets. Thromb Haemost. 1998; 80: 836–839.[Medline] [Order article via Infotrieve]
39. Radomski A, Jurasz P, Sanders EJ, Overall CM, Bigg HF, Edwards DR, Radomski M. W. Identification, regulation and role of tissue inhibitor of metalloproteinases-4 (TIMP-4) in human platelets. Br J Pharmacol. 2002; 137: 1330–1338.[CrossRef][Medline] [Order article via Infotrieve]
40. Goll DE, Thompson VF, Li H, Wei WEI, Cong J. The calpain system. Physiol Rev. 2003; 83: 731–801.
41. Franco SJ, Huttenlocher A. Regulating cell migration: calpains make the cut. J Cell Sci. 2005; 118: 3829–3838.
42. Tompa P, Buzder-Lantos P, Tantos A, Farkas A, Szilagyi A, Banoczi Z, Hudecz F, Friedrich P. On the sequential determinants of calpain cleavage. J Biol Chem. 2004; 279: 20775–20785.
43. Wang KK, Villalobo A, Roufogalis BD. Calmodulin-binding proteins as calpain substrates. Biochem J. 1989; 262: 693–706.[Medline] [Order article via Infotrieve]
44. Satish L, Blair HC, Glading A, Wells A. Interferon-inducible protein 9 (CXCL11)-induced cell motility in keratinocytes requires calcium flux-dependent activation of µ-calpain. Mol Cell Biol. 2005; 25: 1922–1941.
45. Taylor RG, Christiansen JA, Goll DE. Immunolocalization of the calpains and calpastatin in human and bovine platelets. Biomed Biochim Acta. 1991; 50: 491–498.[Medline] [Order article via Infotrieve]
46. Croall DE, DeMartino GN. Calcium-activated neutral protease (calpain) system: structure, function, and regulation. Physiol Rev. 1991; 71: 813–847.
47. Schoenwaelder SM, Yuan Y, Cooray P, Salem HH, Jackson SP. Calpain cleavage of focal adhesion proteins regulates the cytoskeletal attachment of integrin IIbß3 (platelet glycoprotein IIb/IIIa) and the cellular retraction of fibrin clots. J Biol Chem. 1997; 272: 1694–1702.
48. Fox JE. Cytoskeletal proteins and platelet signalling. Thromb Haemost. 2001; 86: 198–213.[Medline] [Order article via Infotrieve]
49. Bialkowska K, Kulkarni S, Du X, Goll DE, Saido TC, Fox JE. Evidence that ß3 integrin-induced Rac activation involves the calpain-dependent formation of integrin clusters that are distinct from the focal complexes and focal adhesions that form as Rac and RhoA become active. J Cell Biol. 2000; 151: 685–696.
50. Du X, Saido TC, Tsubuki S, Indig FE, Williams MJ, Ginsberg MH. Calpain cleavage of the cytoplasmic domain of the integrin ß3 subunit. J Biol Chem. 1995; 270: 26146–26151.
51. Xi X, Bodnar RJ, Li Z, Lam SC, Du X. Critical roles for the COOH-terminal NITY and RGT sequences of the integrin ß3 cytoplasmic domain in inside-out and outside-in signalling. J Cell Biol. 2003; 162: 329–339.
52. Pfaff M, Du X, Ginsberg MH. Calpain cleavage of integrin ß cytoplasmic domains. FEBS Lett. 1999; 460: 17–22.[CrossRef][Medline] [Order article via Infotrieve]
53. Wong MX, Harbour SN, Wee JL, Lau LM, Andrews RK, Jackson DE. Proteolytic cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is regulated by a calmodulin-binding motif. FEBS Lett. 2004; 568: 70–78.[CrossRef][Medline] [Order article via Infotrieve]
54. Naganuma Y, Satoh K, Yi Q, Asazuma N, Yatomi Y, Ozaki Y. Cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1) in platelets exposed to high shear stress. J Thromb Haemost. 2004; 2: 1998–2008.[CrossRef][Medline] [Order article via Infotrieve]
55. Inoue O, Suzuki-Inoue K, McCarty OJ, Moroi M, Ruggeri ZM, Kunicki TJ, Ozaki Y, Watson SP. Laminin stimulates spreading of platelets through integrin 6ß1-dependent activation of GPVI. Blood. 2006; 107: 1405–1412.
56. Farndale RW, Sixma JJ, Barnes MJ, de Groot PG. The role of collagen in thrombosis and hemostasis. J Thromb Haemost. 2004; 2: 561–573.[CrossRef][Medline] [Order article via Infotrieve]
57. Polgar J, Clemetson JM, Kehrel BE, Wiedemann M, Magnenat EM, Wells TN, Clemetson KJ. Platelet activation and signal transduction by convulxin, a C-type lectin from Crotalus durissus terrificus (tropical rattlesnake) venom via the p62/GPVI collagen receptor. J Biol Chem. 1997; 272: 13576–13583.
58. Andrews RK, Gardiner EE, Asazuma N, Berlanga O, Tulasne D, Nieswandt B, Smith AI, Berndt MC, Watson SP. A novel viper venom metalloproteinase, alborhagin, is an agonist at the platelet collagen receptor GPVI. J Biol Chem. 2001; 276: 28092–28097.
59. Liu J, Ware J, Jackson CW, Gartner TK. FcR-chain signals in the absence of GPVI. J Thromb Haemost. 2006; 5: 201–203.[CrossRef]
60. Bori-Sanz T, Suzuki-Inoue K, Berndt MC, Watson SP, Tulasne D. Delineation of the region in the glycoprotein VI tail required for association with the Fc receptor -chain. J Biol Chem. 2003; 278: 35914–35922.
61. Locke D, Liu C, Peng X, Chen H, Kahn ML. Fc R-independent signalling by the platelet collagen receptor glycoprotein VI. J Biol Chem. 2003; 278: 15441–15448.
62. Suzuki-Inoue K, Tulasne D, Bori-Sanz T, Inoue O, Jung SM, Moroi M, Shen Y, Andrews RK, Berndt MC, Watson SP. Association of fyn and lyn with the proline rich domain of GPVI regulates intracellular signalling. J Biol Chem. 2002; 277: 21561–21566.
63. Andrews RK, Suzuki-Inoue K, Shen Y, Tulasne D, Watson SP, Berndt MC. Interaction of calmodulin with the cytoplasmic domain of platelet glycoprotein VI. Blood. 2002; 99: 4219–4221.
64. Gardiner EE, Karunakaran D, Shen Y, Andrews RK, Berndt MC. Controlled shedding of platelet glycoprotein (GP)VI and GPIb-IX-V by ADAM family metalloproteinases. J Thromb Haemostas. 2007;5:in press.
65. Luo S-Z, Mo X, Afshar-Kharghan V, Srinivasan S, López JA, Li R Glycoprotein Ib forms disulfides with two glycoprotein Ibß subunits in the resting platelet. Blood. 2006; 109: 603–609.[CrossRef][Medline]
[Order article via Infotrieve]
66. Coller BS, Kalomiris E, Steinberg M, Scudder LE. Evidence that glycocalicin circulates in normal plasma. J Clin Invest. 1984; 73: 794–799.[Medline] [Order article via Infotrieve]
67. Andrews RK, Munday AD, Mitchell CA, Berndt MC. Interaction of calmodulin with the cytoplasmic domain of the platelet membrane glycoprotein Ib-IX-V complex. Blood. 2001; 98: 681–687.
68. Jurk K, Clemetson KJ, de Groot PG, Brodde MF, Steiner M, Savion N, Varon D, Sixma JJ, Van Aken H, Kehrel BE. Thrombospondin-1 mediates platelet adhesion at high shear via glycoprotein Ib(GPIb): an alternative/backup mechanism to von Willebrand factor. FASEB J. 2003; 17: 1490–1492.
69. Romo GM, Dong J-F, Schade AJ, Gardiner EE, Kansas GS, Li CQ, McIntire LV, Berndt MC, Lopez JA. The glycoprotein Ib-IX-V complex is a platelet counterreceptor for P-selectin. J Exp Med. 1999; 190: 803–814.
70. Dong J-F. Cleavage of ultra-large von Willebrand factor by ADAMTS-13 under flow conditions. J Thromb Haemost. 2005; 3: 1710–1716.[CrossRef][Medline] [Order article via Infotrieve]
71. Padilla A, Moake JL, Bernardo A, Ball C, Wang Y, Arya M, Nolasco L, Turner N, Berndt MC, Anvari B, Lopez JA, Dong JF. P-selectin anchors newly released ultra-large von Willebrand factor multimers to the endothelial cell surface. Blood. 2004; 103: 2150–2156.
72. Simon DI, Chen Z, Xu H, Li CQ, Dong J, McIntire LV, Ballantyne CM, Zhang L, Furman MI, Berndt MC, Lopez JA. Platelet glycoprotein Ib is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18). J Exp Med. 2000; 192: 193–204.
73. Ni H, Denis CV, Subbarao S, Degen JL, Sato TN, Hynes RO, Wagner DD. Persistence of platelet thrombus formation in arterioles of mice lacking both von Willebrand factor and fibrinogen. J Clin Invest. 2000; 106: 385–392.[Medline] [Order article via Infotrieve]
74. Bergmeier W, Piffath CL, Goerge T, Cifuni SM, Ruggeri ZM, Ware J, Wagner DD. The role of platelet adhesion receptor GPIb far exceeds that of its main ligand, von Willebrand factor, in arterial thrombosis. Proc Natl Acad Sci U S A. 2006; 103: 16900–16905.
75. Baglia FA, Badellino KO, Li CQ, Lopez JA, Walsh PN. Factor XI binding to the platelet glycoprotein Ib-IX-V complex promotes factor XI activation by thrombin. J Biol Chem. 2002; 277: 1662–1668.
76. Yun TH, Baglia FA, Myles T, Navaneetham D, Lopez JA, Walsh PN, Leung LL. Thrombin activation of factor XI on activated platelets requires the interaction of factor XI and platelet glycoprotein Ib with thrombin anion-binding exosites I and II, respectively. J Biol Chem. 2003; 278: 48112–48119.
77. Bradford HN, Pixley RA, Colman RW. Human factor XII binding to the glycoprotein Ib-IX-V complex inhibits thrombin-induced platelet aggregation. J Biol Chem. 2000; 275: 22756–22763.
78. Joseph K, Nakazawa Y, Bahou WF, Ghebrehiwet B, Kaplan AP. Platelet glycoprotein Ib: a zinc-dependent binding protein for the heavy chain of high-molecular-weight kininogen. Mol Med. 1999; 5: 555–563.[Medline] [Order article via Infotrieve]
79. Dumas JJ, Kumar R, Seehra J, Somers WS, Mosyak L. Crystal structure of the GPIb-thrombin complex essential for platelet aggregation. Science. 2003; 301: 222–226.
80. Celikel R, McClintock RA, Roberts JR, Mendolicchio GL, Ware J, Varughese KI, Ruggeri ZM. Modulation of -thrombin function by distinct interactions with platelet glycoprotein Ib. Science. 2003; 301: 218–221.
81. Adam F, Bouton MC, Huisse MG, Jandrot-Perrus M. Thrombin interaction with platelet membrane glycoprotein Ib. Trends Mol Med. 2003; 9: 461–464.[CrossRef][Medline]
[Order article via Infotrieve]
82. Vanhoorelbeke K, Ulrichts H, Romijn RA, Huizinga EG, Deckmyn H. The GPIb-thrombin interaction: far from crystal clear. Trends Mol Med. 2004; 10: 33–39.[CrossRef][Medline]
[Order article via Infotrieve]
83. De Cristofaro R, De Candia E. Thrombin domains: structure, function and interaction with platelet receptors. J Thromb Thrombolysis. 2003; 15: 151–163.[CrossRef][Medline] [Order article via Infotrieve]
84. De Candia E, Hall SW, Rutella S, Landolfi R, Andrews RK, De Cristofaro R. Binding of thrombin to glycoprotein Ib accelerates the hydrolysis of Par-1 on intact platelets. J Biol Chem. 2001; 276: 4692–4698.
85. Ramakrishnan V, DeGuzman F, Bao M, Hall SW, Leung LL, Phillips DR. A thrombin receptor function for platelet glycoprotein Ib-IX unmasked by cleavage of glycoprotein V. Proc Natl Acad Sci U S A. 2001; 98: 1823–1828.
86. Moog S, Mangin P, Lenain N, Strassel C, Ravanat C, Schuhler S, Freund M, Santer M, Kahn M, Nieswandt B, Gachet C, Cazenave JP, Lanza F. Platelet glycoprotein V binds to collagen and participates in platelet adhesion and aggregation. Blood. 2001; 98: 1038–1046.
87. Baker J, Griggs RK, Falati S, Poole AW. GPIb potentiates GPVI-induced responses in human platelets. Platelets. 2004; 15: 207–214.[CrossRef][Medline] [Order article via Infotrieve]
88. Sullam PM, Hyun WC, Szollosi J, Dong J, Foss WM, Lopez JA. Physical proximity and functional interplay of the glycoprotein Ib-IX-V complex and the Fc receptor FcRIIA on the platelet plasma membrane. J Biol Chem. 1998; 273: 5331–5336.
89. Shi T, Giannakopoulos B, Yan X, Yu P, Berndt MC, Andrews RK, Rivera J, Iverson GM, Cockerill KA, Linnik MD, Krilis SA. Anti-ß2-glycoprotein I antibodies in complex with ß2-glycoprotein I can activate platelets in a dysregulated manner via glycoprotein Ib-IX-V. Arthritis Rheum. 2006; 54: 2558–2567.[CrossRef][Medline] [Order article via Infotrieve]
90. Wee JL, Jackson DE. The Ig-ITIM superfamily member PECAM-1 regulates the "outside-in" signalling properties of integrin IIbß3 in platelets. Blood. 2005; 106: 3816–3823.
91. Rathore V, Stapleton MA, Hillery CA, Montgomery RR, Nichols TC, Merricks EP, Newman DK, Newman PJ. PECAM-1 negatively regulates GPIb/V/IX signalling in murine platelets. Blood. 2003; 102: 3658–3664.
92. Falati S, Patil S, Gross PL, Stapleton M, Merrill-Skoloff G, Barrett NE, Pixton KL, Weiler H, Cooley B, Newman DK, Newman PJ, Furie BC, Furie B, Gibbins JM. Platelet PECAM-1 inhibits thrombus formation in vivo. Blood. 2006; 107: 535–541.
93. Thai le M, Ashman LK, Harbour SN, Hogarth PM, Jackson DE. Physical proximity and functional interplay of PECAM-1 with the Fc receptor FcRIIa on the platelet plasma membrane. Blood. 2003; 102: 3637–3645.
94. Zhu L, Bergmeier W, Wu J, Jiang H, Stalker TJ, Cieslak M, Fan R, Boumsell L, Kumanogoh A, Kikutani H, Tamagnone L, Wagner DD, Milla ME, Brass LF. Regulated surface expression and shedding support a dual role for semaphoring 4D in platelet responses to vascular injury. Proc Natl Acad Sci U S A. 2007;in press.
95. Andre P, Prasad KS, Denis CV, He M, Papalia JM, Hynes RO, Phillips DR, Wagner DD. CD40L stabilizes arterial thrombi by a ß3 integrin– dependent mechanism. Nat Med. 2002; 8: 247–252.[CrossRef][Medline] [Order article via Infotrieve]
96. Andre P, Nannizzi-Alaimo L, Prasad SK, Phillips DR. Platelet-derived CD40L: the switch-hitting player of cardiovascular disease. Circulation. 2002; 106: 896–899.
97. Ludeman MJ, Zheng YW, Ishii K, Coughlin SR. Regulated shedding of PAR1 N-terminal exodomain from endothelial cells. J Biol Chem. 2004; 279: 18592–18599.
98. Arthur JF, Shen Y, Mu FT, Leon C, Gachet C, Berndt MC, Andrews RK. Calmodulin interacts with the platelet ADP receptor P2Y1. Biochem J. 2006; 398: 339–343.[CrossRef][Medline] [Order article via Infotrieve]
99. Shattil SJ, Newman PJ. Integrins: dynamic scaffolds for adhesion and signalling in platelets. Blood. 2004; 104: 1606–1615.
100. Evans BJ, McDowall A, Taylor PC, Hogg N, Haskard DO, Landis RC. Shedding of lymphocyte function-associated antigen-1 (LFA-1) in a human inflammatory response. Blood. 2006; 107: 3593–3599.
101. Hemler ME. Shedding of heterodimeric leukocyte integrin. Blood. 2006; 107: 3417–3418.
102. Langer H, May AE, Bultmann A, Gawaz M. ADAM15 is an adhesion receptor for platelet GPIIb-IIIa and induces platelet activation. Thromb Haemost. 2005; 94: 555–561.[Medline] [Order article via Infotrieve]
103. Nurden AT, Nurden P. Advantages of fast-acting ADP receptor blockade in ischemic heart disease. Arterioscler Thromb Vasc Biol. 2003; 23: 158–159.
104. Lenain N, Freund M, Leon C, Cazenave J. P, Gachet C. Inhibition of localized thrombosis in P2Y1-deficient mice and rodents treated with MRS2179, a P2Y1 receptor antagonist. J Thromb Haemost. 2003; 1: 1144–1149.[CrossRef][Medline] [Order article via Infotrieve]
105. Massberg S, Gawaz M, Gruner S, Schulte V, Konrad I, Zohlnhofer D, Heinzmann U, Nieswandt B. A crucial role of glycoprotein VI for platelet recruitment to the injured arterial wall in vivo. J Exp Med. 2003; 197: 41–49.
106. McLaughlin S, Murray D. Plasma membrane phosphoinositide organization by protein electrostatics. Nature. 2005; 438: 605–611.[CrossRef][Medline] [Order article via Infotrieve]
107. Alberio L, Friese P, Clemetson KJ, Dale GL. Collagen response and glycoprotein VI function decline progressively as canine platelets age in vivo. Thromb Haemost. 2002; 88: 510–516.[Medline] [Order article via Infotrieve]
108. Thattaliyath B, Cykowski M, Jagadeeswaran P. Young thrombocytes initiate the formation of arterial thrombi in zebrafish. Blood. 2005; 106: 118–124.
109. Nieswandt B, Bergmeier W, Schulte V, Rackebrandt K, Gessner JE, Zirngibl H. Expression and function of the mouse collagen receptor glycoprotein VI is strictly dependent on its association with the FcR chain. J Biol Chem. 2000; 275: 23998–24002.
110. Boylan B, Berndt MC, Kahn ML, Newman PJ. Activation-independent, antibody-mediated removal of GPVI from circulating human platelets: development of a novel NOD/SCID mouse model to evaluate the in vivo effectiveness of anti-human platelet agents. Blood. 2006; 108: 908–914.
111. Boylan B, Chen H, Rathore V, Paddock C, Salacz M, Friedman KD, Curtis BR, Stapleton M, Newman DK, Kahn ML, Newman PJ. Anti-GPVI-associated ITP: an acquired platelet disorder caused by autoantibody-mediated clearance of the GPVI/FcR-chain complex from the human platelet surface. Blood. 2004; 104: 1350–1355.
112. Autelitano DJ, Rajic A, Smith AI, Berndt MC, Ilag LL, Vadas M. The cryptome: a subset of the proteome, comprising cryptic peptides with distinct bioactivities. Drug Discov Today. 2006; 11: 306–314.[CrossRef][Medline] [Order article via Infotrieve]
113. Krstenansky JL, Wang J, Chenail GR, Tiffany MR, Kuesters GM, Natke BC, Nestor Jr JL. Probing proteinase active sites using oriented peptide mixture libraries – ADAM-10. Lett Drug Design Disc. 2004; 1: 6–13.[CrossRef]
114. Goto S, Ikeda Y, Saldivar E, Ruggeri ZM. Distinct mechanisms of platelet aggregation as a consequence of different shearing flow conditions. J Clin Invest. 1998; 101: 479–486.[Medline] [Order article via Infotrieve]
115. Goto S, Tamura N, Handa S, Arai M, Kodama K, Takayama H. Involvement of glycoprotein VI in platelet thrombus formation on both collagen and von Willebrand factor surfaces under flow conditions. Circulation. 2002; 106: 266–272.
116. Maxwell MJ, Westein E, Nesbitt WS, Giuliano S, Dopheide SM, Jackson SP. Identification of a 2-stage platelet aggregation process mediating shear-dependent thrombus formation. Blood. 2007; 109: 566–576.
117. Kulkarni S, Dopheide SM, Yap CL, Ravanat C, Freund M, Mangin P, Heel KA, Street A, Harper IS, Lanza F, Jackson SP. A revised model of platelet aggregation. J Clin Invest. 2000; 105: 783–791.[Medline] [Order article via Infotrieve]
118. Ni H, Yuen PS, Papalia JM, Trevithick JE, Sakai T, Fassler R, Hynes RO, Wagner DD. Plasma fibronectin promotes thrombus growth and stability in injured arterioles. Proc Natl Acad Sci U S A. 2003; 100: 2415–2419.
119. Denis C, Methia N, Frenette PS, Rayburn H, Ullman-Cullere M, Hynes RO, Wagner DD. A mouse model of severe von Willebrand disease: defects in hemostasis and thrombosis. Proc Natl Acad Sci U S A. 1998; 95: 9524–9529.
120. Falati S, Gross P, Merrill-Skoloff G, Furie BC, Furie B. Real-time in vivo imaging of platelets, tissue factor and fibrin during arterial thrombus formation in the mouse. Nat Med. 2002; 8: 1175–1181.[CrossRef][Medline] [Order article via Infotrieve]
121. Celi A, Merrill-Skoloff G, Gross P, Falati S, Sim DS, Flaumenhaft R, Furie BC, Furie B. Thrombus formation: direct real-time observation and digital analysis of thrombus assembly in a living mouse by confocal and widefield intravital microscopy. J Thromb Haemost. 2003; 1: 60–68.[CrossRef][Medline] [Order article via Infotrieve]
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