Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin α2β1. These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin αIIbβ3, a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins α2β1 and αIIbβ3 contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.
Platelets are small, anucleate blood cells derived from megakaryocytes. They provide a first line of defence following injury, forming thrombi that patch-up damaged tissue and thereby playing an indispensable role in haemostasis. However, inappropriate platelet activation can lead to thrombosis, myocardial infarction and strokes. Platelets are also believed to be involved in the development of atherosclerosis in coronary or carotid arteries, which is commonly the trigger for thrombosis.
Platelets possess several cell-surface receptors that allow them to adhere to sites of tissue damage and spread to form a monolayer of cells that covers the exposed tissue. Spreading is accompanied by the secretion or synthesis of several prothrombotic factors, such as ADP, serotonin and thromboxane A2, which act in an autocrine/paracrine fashion and activate or prime approaching platelets (Ruggeri, 2002). During platelet activation, inside-out signalling upregulates the affinity of several platelet integrins, including integrin αIIbβ3 (Calderwood, 2004; Liddington and Ginsberg, 2002; Shattil et al., 1998). This binds to the bivalent ligand fibrinogen, which is present in the plasma and is released by activated platelets. The resulting platelet aggregation leads to the assembly of a platelet thrombus.
Although platelets do not directly activate the coagulation pathways, they are vital for effective blood coagulation, providing a surface for the assembly of the prothrombinase complex. This `procoagulant' property depends on exposure of aminophospholipids such as phosphatidylserine on the cell surface, and the release of phospholipid microparticles (Heemskerk et al., 2002). Coagulation is precipitated by the cleavage of fibrinogen by the serine protease thrombin, an end-product of the clotting pathways, and subsequent formation of an insoluble polymeric fibrin mesh. Since thrombin is generated on the surface of activated platelets, fibrin is deposited in the platelet thrombus as it is assembled (Falati et al., 2002). Thrombin is also a powerful platelet agonist that can stimulate platelet aggregation and thrombus formation.
Here, I discuss our current understanding of the initial signalling responses of platelets following tissue injury, focusing on signalling stimulated by non-integrin adhesion receptors and mechanisms for the negative regulation of platelet function by platelet endothelial cell adhesion molecule 1 (PECAM-1). Reviews that cover other aspects of platelet signalling may be found elsewhere (Jackson et al., 2003; Nieswandt and Watson, 2003).
Platelet adhesion receptors
When the integrity of the vascular system is breached, platelets are exposed to components of the extracellular matrix (ECM) present in the blood vessel wall and beyond. Platelets can interact directly or indirectly with several ECM proteins, but of principal importance are the collagens (Farndale et al., 2004). Humans possess at least 25 forms of collagen (Hashimoto et al., 2002), and several of these (I, III, IV, V, VI, VIII, XII, XIII and XIV) are present in the blood vessel wall (Barnes and Farndale, 1999). In addition, type IV collagen is present in the subendothelial basement membrane.
The GPIb-V-IX complex
The initial entrapment of platelets on subendothelial collagens requires the plasma protein von Willebrand factor (VWF), which under the shear stress conditions present in arteries and small arterioles binds simultaneously to collagen and the platelet glycoprotein (GP) complex GPIb-V-IX, or to the integrin αIIbβ3 in its activated conformation (Fig. 1) (Alevriadou et al., 1993; Ruggeri, 1997; Savage et al., 1996; Sixma et al., 1997). VWF-dependent interactions have a fast off rate and cannot support the assembly of a platelet thrombus. These interactions are superseded by more-stable binding of collagen to platelet collagen receptor, principally integrin α2β1 and GPVI (Moroi et al., 1996; Saelman et al., 1994b; Staatz et al., 1989) (reviewed by Farndale et al., 2004; Jackson et al., 2003; Nieswandt and Watson, 2003; Ruggeri, 2002).
Binding of VWF to GPIb-V-IX upregulates integrin αIIbβ3 affinity (Asazuma et al., 1997; Kasirer-Friede et al., 2004; Milner et al., 1998; Munday et al., 2000; Nesbitt et al., 2002; Torti et al., 1999; Yuan et al., 1999). The integrin can then bind to VWF, thereby enhancing adhesion, and contributing to thrombus formation by binding to fibrinogen.
Integrin α2β1 was the first platelet collagen receptor to be identified and binds to collagen in a Mg2+-dependent manner (Kunicki et al., 1988; Nieuwenhuis et al., 1985; Santoro, 1986; Santoro et al., 1988; Sixma et al., 1995; Sixma et al., 1997). Integrin α2β1 does not stimulate tyrosine kinase activity, which is required for collagen-induced platelet activation. Santoro et al. therefore proposed a two-site–two-step model of platelet activation (Santoro et al., 1991) in which integrin α2β1 stabilizes interactions with collagen, allowing it to interact with a second collagen receptor that can activate tyrosine-kinase-dependent signalling. This model, in which platelet adhesion and activation are considered distinct events, is supported by more-recent studies (Jung and Moroi, 1998; Jung and Moroi, 2000; Moroi et al., 2000; Siljander et al., 2004).
Glycoprotein VI (GPVI)
The second collagen receptor is a multi-protein complex containing GPVI and the Fc receptor γ-chain (FcRγ)* (Gibbins et al., 1997; Tsuji et al., 1997). GPVI was originally identified as a potential collagen receptor in patients expressing low levels of the protein, who display mild bleeding diatheses (Arai et al., 1995; Moroi et al., 1989; Ryo et al., 1992), and was subsequently cloned and characterized (Clemetson et al., 1999; Ezumi et al., 2000; Jandrot-Perrus et al., 2000). It associates noncovalently with FcRγ (Gibbins et al., 1997; Tsuji et al., 1997), and anti-GPVI F(ab')2 fragments stimulate platelet activation (Gibbins et al., 1997; Sugiyama et al., 1987). Studies of knockout or knockdown mice support the idea that this receptor is the principal activatory collagen receptor (Kato et al., 2003; Massberg et al., 2003; Nieswandt et al., 2001a; Schulte et al., 2003). Mutagenesis studies based on sequence differences between human and mouse GPVI, and the generation of an inhibitory phage antibody against GPVI, have enabled the collagen-binding surface in GPVI to be defined (Smethurst et al., 2004).
The FcRγ is a component of the multi-subunit high-affinity receptor for immunoglobulin (Ig)E, FcϵRI (Blank et al., 1989; Kuster et al., 1990), and the IgG receptors FcγRI and FcγRIII (van de Winkel and Capel, 1993). The FcRγ is noncovalently associated with these receptors and contains an immunoreceptor tyrosine-based activation motif (ITAM) (Reth, 1989), which becomes phosphorylated on receptor ligation and clustering. The IgG receptor FcγRIIA does not couple to FcRγ but contains a similar ITAM in its cytoplasmic tail (van de Winkel and Capel, 1993).
The complementary roles of α2β1 and GPVI
Much recent discussion has focused on the relative contributions of α2β1 and GPVI in platelet adhesion to collagen and to thrombus formation. Mouse platelets that lack GPVI as a consequence of intravenous injection of an anti-mouse GPVI monoclonal antibody or FcRγ gene ablation [GPVI expression is dependent on FcRγ expression (Nieswandt et al., 2000)] are resistant to activation by collagen and collagen-related peptide (which is a GPVI selective ligand, see below) (Nieswandt et al., 2001b) or adhesion to collagen under static and flow conditions (Nieswandt et al., 2001a). These mice are resistant to lethal pulmonary thromboembolism induced by collagen and adrenaline infusion, and tail bleeding times are moderately extended (Nieswandt et al., 2001b). Integrin-β1-null platelets have been reported to aggregate in response to collagen, although this is slightly delayed (Nieswandt et al., 2001a). At low (150 s–1) and high (1000 s–1) levels of shear stress, they adhere to soluble collagen normally. The authors of these studies have therefore proposed that GPVI, and not integrin α2β1, could thus be essential for the platelet adhesion to collagen, and that the initial collagen interaction is predominantly through GPVI, which results in cell signalling that upregulates α2β1 affinity (Nieswandt et al., 2001a). However, in contrast to β1-deficient-platelets, α2-deficient platelets fail to adhere to fibrillar collagen under low shear stress (Chen et al., 2002) and abnormal interaction of α2-deficient platelets with soluble collagen has been reported (Holtkotter et al., 2002).
Transgenic mice in which GPVI is deleted have been generated recently, and platelets from these mice fail to respond to collagen in aggregation assays (Kato et al., 2003). However, tail bleeding times in these mice are essentially normal. Moreover, in perfusion experiments at high shear stress, platelets lacking GPVI adhere to insoluble type I collagen but fail to form thrombi. These results closely resemble those obtained for human GPVI-deficient platelets in similar assays (Moroi et al., 1996). Similar observations have also been reported for normal platelets and a function-blocking anti-GPVI antibody (Siljander et al., 2004), suggesting that GPVI and α2β1 might play complementary roles in which α2β1 is able to bind to collagen before platelet activation and GPVI is required for thrombus formation (Kuijpers et al., 2003).
The reasons for discrepancies between some of these studies remain to be resolved, although these are likely to be due to different experimental systems used. However, the studies do point towards a pivotal role of GPVI in haemostasis and thrombosis. Since inhibition and in vivo depletion of GPVI are well tolerated in mice, targeting GPVI might provide new avenues for anti-thrombotic therapies.
More platelet collagen receptors?
A third functional collagen receptor on platelets might also exist. For example, in platelets lacking the GPVI-FcRγ complex, collagen stimulates low levels of protein tyrosine phosphorylation. Furthermore, in the presence of α2β1- and GPV-blocking antibodies, mouse platelets that possess approximately 20% of the normal levels of GPVI-FcRγ respond to collagen but not to collagen-related peptide (a GPVI-selective ligand, see below) (Poole et al., 1997; Snell et al., 2002). Several additional putative platelet collagen receptors have been reported, including 68 kDa (Monnet et al., 2001; Monnet and Fauvel-Lefeve, 2000) and 65 kDa (Chiang et al., 1997) binding proteins for type III collagen, and a 47 kDa type I collagen-binding species (Chiang et al., 2002). It is unclear whether any of these contribute to platelet signalling, although some may have supporting roles and modulate GPVI-mediated responses. CD36 was proposed to be a platelet collagen receptor, but since CD36-deficient platelets exhibit normal responses to collagen, this is unlikely (Daniel et al., 1994; Saelman et al., 1994a; Yamamoto et al., 1992). The most compelling evidence for a third collagen receptor is from studies of platelets from GPV-null mice, which display decreased adhesion and aggregation responses to collagen (Kahn et al., 1999; Ramakrishnan et al., 1999). Such platelets curiously demonstrate enhanced responses to thrombin (Ramakrishnan et al., 1999) and therefore determination of the relevance in vivo of GPV in collagen responses has not been possible.
Cell signalling mechanisms
Since platelets are anucleate, most in vitro molecular biology techniques are not applicable to these cells. Although the production of platelet-like particles from megakaryocytes in culture is possible, this is not routinely achieved and the low yield prevents extensive functional and biochemical analysis (Hartwig and Italiano, 2003; Italiano et al., 1999). Much research performed on primary platelets has used biochemical techniques, selective inhibitors of signalling enzymes, examination of naturally occurring mutations, and comparison with signalling mechanisms in other cells. In recent years, transgenic mouse models have become increasingly important.
Signalling through GPIb
As described below, the initial interaction of the GPIb-V-IX complex with VWF stimulates platelet signalling, which leads to the secretion of granules and the upregulation of integrin affinity. VWF binding requires shear stress, but shear-induced conditions for GPIb-VWF binding appear to be mimicked by the conformational modulator botrocetin and the antibiotic ristocetin, molecules that have been used in many GPIb-V-IX signalling studies. However, several studies have examined GPIb-V-IX in vitro using flow-based assay systems (Nesbitt et al., 2002; Yap et al., 2000). GPIb binding is associated with the stimulation of tyrosine kinase signalling (Asazuma et al., 1997; Ozaki et al., 1995; Razdan et al., 1994). Principal players in this pathway include the non-receptor tyrosine kinases Src, Fyn, Lyn and Syk, phospholipase Cγ2 (PLCγ2), and adaptor proteins such as Shc, linker for activation of T cells (LAT) and SLP-76 (Asazuma et al., 1997; Falati et al., 1999; Jackson et al., 1994; Marshall et al., 2002; Torti et al., 1999; Wu et al., 2001). Exactly how these components cooperate in GPIb-V-IX signalling is not known, but there is good evidence that it might be similar to GPVI signalling in platelets (described in detail below; Fig. 2). GPIb might also activate platelets by triggering Src-family-kinase-mediated phosphorylation of FcRγ and FcγRIIA, receptors with which GPIb physically associates (Canobbio et al., 2001; Falati et al., 1999; Sullam et al., 1998; Torti et al., 1999; Wu et al., 2001).
The binding of VWF to GPIb under shear stress can stimulate calcium mobilization (Kroll et al., 1991; Mazzucato et al., 2002; Milner et al., 1998; Nesbitt et al., 2002; Yap et al., 2000), protein kinase C (PKC), protein kinase G (PKG) (Li et al., 2003; Yap et al., 2000), phosphoinositide 3-kinase (PI3K) (Jackson et al., 1994; Munday et al., 2000; Yap et al., 2000) and cytoskeletal rearrangements (Torti et al., 1999; Yuan et al., 1999). VWF binding also upregulates integrin αIIbβ3 affinity indirectly through the stimulation of ADP secretion (Moake et al., 1988). Other studies have indicated that GPIb-V-IX controls upregulation of integrin αIIbβ3 affinity through FcRγ-independent interactions with signalling molecules and sequentially activates Src-family kinases, calcium oscillations, PI3K and PKC. Direct interactions of GPIb-V-IX with molecules such as calmodulin and 14-3-3ζ, and with the platelet cytoskeleton, have also been implicated in this (Andrews et al., 1998; Andrews et al., 2001; Cunningham et al., 1996; Kasirer-Friede et al., 2004; Munday et al., 2000).
For some time, controversy has surrounded the question of whether the GPIb-V-IX complex signals at all, and some studies have failed to show signalling by this receptor complex (Kuwahara et al., 1999). This is likely to reflect differences between studies with respect to the array of preparations and species of VWF, GPIb-V-IX-binding venom proteins and peptides, conformational modulators, experimental strategies and cell types used. Given the increasing literature in this area, it is likely that GPIb-V-IX does signal, although in comparison with what is known about GPVI-mediated signalling (see below), our understanding is much less developed. Although many molecules are implicated in GPIb-V-IX signalling, these have yet to be assembled into a defined signalling pathway.
Signalling through GPVI
A single collagen fibre can bind simultaneously to multiple different receptors and collagen-binding proteins on the platelet surface, which complicates the analysis of signalling by individual receptors. The analysis of differential adhesion and aggregatory properties of cyanogen bromide fragments of collagen led to the realization that different collagen receptors bind to distinct sequences within collagen fibres (Morton et al., 1989; Morton et al., 1994). A combination of collagen peptide functional analysis and the development of methods for the synthesis of triple helical peptides in which to present receptor-binding sequences have enabled the development of GPVI- and α2β1-selective ligands. Collagen-related peptides (CRPs) containing a repeated GPO sequence (single-letter amino acid code; O represents hydroxyproline) bind specifically to GPVI and are highly potent platelet agonists, and a GFOGER peptide supports α2β1-mediated adhesion (Asselin et al., 1997; Kehrel et al., 1998; Morton et al., 1995; Knight et al., 1998). These reagents, together with activatory and inhibitory antibodies to GPVI (Nieswandt et al., 2000; Smethurst et al., 2004; Sugiyama et al., 1987) and α2β1 (Polanowska-Grabowska and Gear, 1992; Stevens et al., 2004), and snake venom proteins, particularly convulxin (Batuwangala et al., 2004; Francischetti et al., 1997), have proven invaluable tools to study collagen-receptor-mediated signalling and function. However, it should be noted that it has recently been reported that, in addition to binding GPVI, convulxin also binds GPIb (Kanaji et al., 2003).
Exposure of platelets to collagen surfaces is believed to result in clustering of GPVI, which triggers the tyrosine phosphorylation of FcRγ (Gibbins et al., 1996; Gibbins et al., 1997; Poole et al., 1997; Tsuji et al., 1997) (Fig. 2). Several reports indicate that GPVI signalling is influenced strongly by glycolipid-enriched microdomains (GEMS, rafts) in the plasma membrane, although there is disagreement between authors on whether GPVI is constitutively associated with, or recruited to, lipid rafts (Ezumi et al., 2002; Locke et al., 2002; Wonerow et al., 2002).
The Src-family tyrosine kinases Fyn and Lyn, which are physically associated with GPVI, are responsible for FcRγ phosphoryation (Briddon and Watson, 1999; Ezumi et al., 1998; Quek et al., 2000; Suzuki-Inoue et al., 2002). They target conserved tyrosine residues within the immunoreceptor tyrosine-based activation motif [ITAM; consensus: Y-X-X-L/I-X6-8-Y-X-X-L/I, where X denotes any amino acid (Reth, 1989)] in the cytoplasmic tail of FcRγ (Gibbins et al., 1996; Poole et al., 1997). The phosphorylated ITAM provides a docking site for Syk, which binds specifically through tandem Src-homology 2 (SH2) domains (Benhamou et al., 1993; Shiue et al., 1995). As a consequence, Syk becomes tyrosine phosphorylated, probably by autophosphorylation, and activated. A substrate for Syk is the adaptor LAT (Zhang et al., 1998), which possesses multiple phosphorylation sites that act as docking sites for recruitment of additional proteins to form a signalling complex (Gibbins et al., 1998; Pasquet et al., 1999b; Sarkar, 1998). For example, PLCγ2 and PI3K (p85/p110) are brought to the vicinity of their substrates at the plasma membrane through interaction with tyrosine-phosphorylated LAT (Gibbins et al., 1998; Gross et al., 1999b).
PI3K generates phosphatidylinositol (3,4)-bisphosphate [PtdIns(3,4)P2] and phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] (Foster et al., 2003), which enable the recruitment to the plasma membrane of proteins that possess specific pleckstrin homology (PH) domains, including PLCγ2 and Btk, a tyrosine kinase that is also associated with Lyn (Pasquet et al., 1999b; Pasquet et al., 2000; Quek et al., 1998). Btk is believed to be partially responsible for the tyrosine phosphorylation of PLCγ2, which becomes activated and generates the second messengers inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] and diacylglycerol (DAG) (Oda et al., 2000; Quek et al., 1998). Ins(1,4,5)P3-mediated mobilization of calcium and DAG-mediated activation of PKC are essential components of the platelet activation process, irrespective of the agonist, and are necessary for platelet secretion and aggregation.
Several other proteins that appear to be important for the regulation of PLCγ2 are recruited to tyrosine-phosphorylated LAT. These include the Syk substrate SLP-76, the loss of which results in reduced tyrosine phosphorylation of PLCγ2 (Gross et al., 1999a; Gross et al., 1999b; Leo et al., 2002). Several adaptor proteins such as Gads, Grb2, Cbl and SLAP-130, and the GTP-exchange factor Vav, are also recruited to the signalling complex (Asazuma et al., 2000; Pearce et al., 2002). The specific roles of these molecules remain to be established, although Vav1-deficient (but not Vav2-deficient) mouse platelets display weakly diminished GPVI-stimulated aggregation responses (Pearce et al., 2002).
PI3K is important for platelet adhesion, spreading and aggregation (Falet et al., 2000; Pasquet et al., 1999a), activating several other signalling molecules, most notably protein kinase B (PKB, also known as Akt) (Barry and Gibbins, 2002; Kroner et al., 2000; Woulfe et al., 2004). Platelets possess two isoforms of PKB: PKBα and PKBβ (Akt1 and Akt2). PKBβ is important for normal platelet function and thrombus formation (Woulfe et al., 2004). PKBβ-null platelets have impaired alpha and dense granule secretion, and also impaired activation of integrin αIIbβ3. A downstream target of PKB is glycogen synthase kinase 3 (GSK3) (Doble and Woodgett, 2003), which is inactivated by phosphorylation (Barry et al., 2003). Since a range of GSK3 inhibitors inhibit platelet aggregation (Barry et al., 2003), GSK3 might have a negative regulatory function in platelets following stimulation. However, it should be noted that GSK3 might be phosphorylated by kinases other than PKB.
Another downstream effector of PI3K in platelets is integrin-linked kinase (ILK), a serine/threonine kinase that interacts with the cytoplasmic tails of β1 and β3 integrin subunits (Hannigan et al., 1996). ILK in platelets is probably important for both outside-in and inside-out signalling by the integrins α2β1 and αIIbβ3 (Pasquet et al., 2002; Stevens et al., 2004; Yamaji et al., 2002). It is speculated that ILK is responsible for the regulation of PKB by phosphorylating Ser473. The other regulatory site in PKB is Thr308, which is phosphorylated by phosphoinositide-dependent kinase 1 (PDK1). This enzyme is present in platelets and, upon platelet activation, forms a ternary complex with PKB and ILK (Barry and Gibbins, 2002).
Many of the early signalling events following stimulation of GPVI have been characterized. However, our knowledge of GPVI signalling is far from complete, and detail of events more distal from the receptor, such as PI3K-dependent signalling, is less well defined. An added layer of complexity is synergism between signalling mechanisms employed by adhesion receptors such as GPIb-V-IX and GPVI, in addition to synergism with secondary soluble agonists secreted from activated platelets. Although many questions remain to be answered, the culmination of platelet activation is inside-out signalling that upregulates the affinity of integrin αIIbβ3 and integrin α2β1, which facilitate thrombus formation.
Integrin αIIbβ3 also exhibits outside-in signalling upon ligation, which enhances the activation process through positive feedback. Integrin αIIbβ3 signalling has been extensively studied and is the subject of several recent reviews to which readers are directed (Calderwood, 2004; Calderwood et al., 2000; Eto et al., 2002; Liddington and Ginsberg, 2002; Phillips et al., 2001a; Phillips et al., 2001b; Tadokoro et al., 2003).
Research focusing on the stimulation of tyrosine kinase signalling in platelets in suspension indicated initially that integrin α2β1 does not engage in outside-in signalling (Hers et al., 2000). However, the use of recently developed α2β1-selective ligands indicates that this is incorrect. The adhesion of platelets to GFOGER peptides is accompanied by the tyrosine phosphorylation of several proteins, including Src, Syk, SLP-76 and PLCγ2, which are also involved in GPVI signalling, and calcium-dependent spreading (Inoue et al., 2003). p38 MAP kinase, ILK, Rac and PAK have also been implicated downstream of α2β1 ligation (Stevens et al., 2004; Sundaresan and Farndale, 2003; Suzuki-Inoue et al., 2001). A more detailed map combining these molecules into an α2β1 signalling pathway has yet to be established, and this is an important focus for future research.
Inhibitory platelet adhesion receptor signalling
Negative regulation of platelets is essential to prevent uncontrolled thrombosis. The roles of nitric oxide (NO) and prostacyclin (PGI2) are well established in the inhibition of platelet function (Geiger, 2001; Radomski et al., 1987). However, platelet activation can also be inhibited by signalling through the adhesion molecule PECAM-1 (also known as CD31) (Cicmil et al., 2002; Jones et al., 2001; Patil et al., 2001). PECAM-1 is expressed on several blood cell types and on endothelial cells, and is associated with the regulation of a range of processes, including trans-endothelial migration of leukocytes, regulation of cell activation and regulation of apoptosis (Newman and Newman, 2003). The functions of PECAM-1 are believed to be mediated by homophilic ligand binding (Albelda et al., 1991), although integrin αvβ3 and CD38 have also been proposed as ligands (Buckley et al., 1996). PECAM-1 dimerization has been shown to support adhesive properties of the molecule, and oligomerization causes cell signalling (Zhao and Newman, 2001).
The first evidence that PECAM-1 regulates the function of platelets in vivo was reported in 1994 in studies showing that time to vascular occlusion was increased following intravenous injection of anti-PECAM-1 antibodies in a mouse vascular injury model (Rosenblum et al., 1994). This was attributed to the inhibition by anti-PECAM-1 antibodies of PECAM-1-mediated platelet adhesion. More-recent studies indicate that this effect might have been due to the stimulation of PECAM-1 signalling. When stimulated through homophilic interactions and/or clustering, PECAM-1 is tyrosine phosphorylated on immunoreceptor tyrosine-based inhibition motifs [ITIMs; consensus: L/I/V/S-X-Y-X-X-L/V (Burshtyn et al., 1997)] in its cytoplasmic tail (Gibbins, 2002; Jackson et al., 1997a) by Src-family kinases. This facilitates the recruitment of tyrosine, serine/threonine or possibly lipid phosphatases, and the consequent inhibition of kinase-dependent signalling (Cicmil et al., 2000; Jackson et al., 1997b; Relou et al., 2003). The protein tyrosine phosphatases Shp-1 and Shp-2 and the serine/threonine protein phosphatase PP2A associate with PECAM-1 in platelets (Jackson et al., 1997b; Relou et al., 2003). Although PECAM-1 signalling reduces total platelet protein tyrosine phosphorylation, inositol phosphate production, calcium mobilization and PI3K signalling (Cicmil et al., 2002; Jones et al., 2001; Thai et al., 2003), specific substrates for these phosphatases in platelets are currently unclear.
PECAM-1 also inhibits GPIb-mediated platelet activation (Rathore et al., 2003) and downregulates FcγRIIA-mediated platelet responses (Thai et al., 2003). The effects of PECAM-1 appear not to be restricted to inhibition of ITAM-mediated signalling: thrombin-dependent and oxidized low-density lipoprotein (LDL)-stimulated platelet signalling are also inhibited (Cicmil et al., 2002; Relou et al., 2003).
Platelet PECAM-1 becomes tyrosine phosphorylated following stimulation of platelets with a range of agonists and upon platelet aggregation, suggesting a negative-feedback role (Cicmil et al., 2000; Jones et al., 2001). Since the principal ligand for PECAM-1 is PECAM-1 itself (Albelda et al., 1991), interactions between PECAM-1 on platelets and endothelial cells might restrict the growth of a thrombus through the feedback mechanisms described above. Indeed, thrombi formed in PECAM-1-null mice are larger and more stable in comparison with those formed in wild-type mice (Falati et al., 2003). The balance between signalling through activatory adhesion receptors and receptors for soluble platelet agonists, and signalling stimulated by PECAM-1 may regulate the stimulus threshold for thrombus formation and may determine thrombus size and stability.
Our understanding of the receptors and signalling mechanisms that regulate thrombus formation has advanced markedly in recent years. The ability of platelets to respond specifically and rapidly to subendothelial proteins exposed upon tissue injury, and under conditions of shear stress, is crucial for effective haemostasis. The identification and characterization of the platelet adhesion receptors GPIb-V-IX and GPVI, and integrins αIIbβ3 and α2β1, are important milestones in our understanding of these processes. The complementary roles of these receptors in platelet adhesion and cell signalling leading to thrombus formation are clearly established. Many of the early signalling events following GPVI ligation are characterized, although our understanding of GPIb-V-IX is less advanced. With many of the spectrum of molecules involved in GPVI and integrin αIIbβ3 signalling also implicated in GPIb-V-IX signalling, rapid progress is anticipated in this area. Similarly, our knowledge of the signalling mechanisms through which PECAM-1 inhibits activatory signals from receptors such as GPVI and GPIb-V-IX lacks depth, and this too is an important area for future development.
The biggest challenge presented to researchers studying platelet biology is to relate the significance of platelet signalling and function in vitro to the in vivo situation of haemostasis and thrombosis. The use of in vivo models of thrombosis, as well as sophisticated methodology to measure platelet signalling and thrombus formation under flow, are important technical developments towards this aim.
The translation of basic research toward new strategies to prevent arterial thrombosis underscores much of the research in this area. New avenues for investigation are presented by the potential benefits of blocking the interactions of VWF with GPIb-V-IX, and collagen with GPVI and/or integrin α2β1, or the activation of PECAM-1, or pharmacological regulation of the signalling mechanisms employed by these receptors. A substantial challenge is the targeting of pathological thrombi yet with minimal side-effects (e.g. haemorrhage). The platelet-specific expression of GPIb-V-IX and GPVI, and the well-tolerated deletion of the gene encoding GPVI in mice, indicate these molecules may underlie future advances in anti-platelet therapy.
We acknowledge valuable discussions with Dr A. Poole (University of Bristol, UK) during the preparation of this manuscript. Research in the author's laboratory is supported by grants from the Medical Research Council, the Biotechnology and Biological Sciences Research Council, the British Heart Foundation and the Wellcome Trust.