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11 Platelets inthePathogenesis ofVascular Disease andTheir Role asaTherapeutic…
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atherosclerotic plaque rupture is the precipitating event leading to vessel occlusion
ultimately causing myocardial infarction or ischaemic stroke—two of the leading
causes of mortality and morbidity globally. Consequently, the use of anti-platelet
drugs is globally one of the most widely applied pharmaceutical therapies.
This chapter will discuss the platelet receptors, soluble agonists and respective G
protein coupled receptors that mediate platelet adhesion, activation and aggregation. The recently discovered proinammatory functions of platelets and how they
participate in the pathogenesis of atherosclerosis will be outlined. Finally, an overview of current and future anti-platelet therapies will be provided in addition to an
outline of the commonly employed platelet function tests.
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11.2 Platelet Structure andFunction
11.2.1 Platelet Adhesion Receptors
In order to carry out their specialized role, platelets have a distinct structure
(Fig. 11.1). The resting platelet has a characteristic ‘discoid’ morphology and
expresses a large number of adhesion receptors on the cell surface to regulate
platelet- extracellular matrix, and platelet-platelet interactions. Further, adhesion
receptors allow the transmission of extracellular signals by activating intracellular
signalling pathways, which in turn can modulate platelet adhesion and activation
responses. The most abundant platelet adhesion receptor is Glycoprotein (GP) IIb/
IIIa (also known as integrin α
ies per platelet [1]. The major ligand of GPIIb/IIIa is brin(ogen) and therefore this
receptor plays an essential role in mediating stable platelet adhesion via interaction
with immobilised brin(ogen) and also platelet aggregation, via soluble brin(ogen)
which acts to crosslink adjacent platelets [2]. Whilst GPIIb/IIIa exists in a low afnity state on the resting platelet, upon platelet activation it undergoes a conformational change (so-called integrin inside-out signalling) such that GPIIb/IIIa exposes
the ligand (brin(ogen)) binding pocket and thus adopts a high afnity conformation towards brin(ogen) [3]. The importance of GPIIb/IIIa in mediating platelet
adhesion and aggregation is underscored by the bleeding phenotype seen in patients
with Glanzmanns thrombasthenia, who exhibit a deciency of GPIIb/IIIa expression or whose GPIIb/IIIa is functionally impaired.
or CD41/CD61) with approximately 80,000 cop-
IIbβ3
11.2.2 Glycoprotein IIb/IIIa (GPIIb/IIIa) Structure
GPIIb/IIIa has a large extracellular portion formed by one α and β subunit with a
single transmembrane spanning region and short cytoplasmic tail (Fig.11.1) The α
subunit, β propeller domain and β3 subunit domain come together to form the
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globular, ligand binding head [3]. The other domains of the α and β subunit, comprise two exible ‘stalks’. As such, the integrin has a bent conformation in the resting state and upon activation extends in a process associated with reorganisation of
the ligand-binding domain [3].
The major ligands of GPIIb/IIIa are brinogen, von Willebrand factor (vWF) and
bronectin, which all bind to the extracellular globular head [4]. A common feature
of all GPIIb/IIIa ligands is the presence of arginine-glycine-aspartic acid (RGD)
motifs, which are recognised by the ligand binding site of GPIIb/IIIa [5]. The RGD
sequence is contained within the α chain of brinogen. However, brinogen, the
major GPIIb/IIIa ligand binds to the integrin via the C terminus of the brinogen γ
chain, which lacks an RGD motif [5]. Whether RGD sequences or the α chain of
brinogen bind to similar or distinct regions of the integrin remains unresolved.
GPIIb/IIIa has a low afnity for its ligands in the resting conformation [6]. However,
upon platelet stimulation, the integrin extends and opens the globular head region,
thus allowing more efcient ligand binding between the propeller domain and βA
domain [7]. (Figs.11.1 and 11.2) Ligand binding itself may also induce conformational changes in GPIIb/IIIa.
Fig. 11.2 GPIIb/IIIa inside out signalling. A schematic representation of GPIIb/IIIa inside-out
signalling events leading to the activation of GPIIb/IIIa. Activation of platelets by G-protein
coupled receptors (GPCR), leads to the activation of phospholipase C (PLC) leading to the
mobilisation of intracellular calcium and activation of protein kinase C (PKC). These signals
result in the activation of the Guanine nucleotide exchange factor CalDAG-GEF1, ultimately
leading to activation of the small GTPase Rap1b. Rap1b mediates GPIIb/IIIa activation by forming a complex with RIAM, which is thought to activate and localise talin to the plasma membrane. Binding of talin to the integrin disrupts the salt bridge between the cytoplasmic tails of
GPIIb/IIIa, thus allowing long range conformational changes of the extracellular integrin
domains. Kindlins, a family of FERM domain containing proteins, bind to integrin β-cytoplasmic
tails and appear to play an essential role in integrin inside out signalling. PI3Kβ, activated downstream of G
GPIIb/IIIa activation
linked signals, plays an important role in Rap1b activation and therefore sustains
i
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11.2.3 Glycoprotein GPIIb/IIIIa Activation
Although the cytoplasmic tails of GPIIb/IIIa are short, they play a fundamental role
in regulating GPIIb/IIIa signalling and adhesive function [8]. The ability of signalling events at the cytoplasmic portion of the integrin to induce long range conformational changes of the extracellular domains such that GPIIb/IIIa switches from a low
afnity to high, is referred to as “inside-out signalling”. Whilst the cytoplasmic tails
have no intrinsic signalling activity, this function is carried out by the over 20 proteins that have been identied to bind to the cytoplasmic tail of GPIIb/IIIa to facilitate bidirectional signals [9]. Importantly, these proteins also link GPIIb/IIIa to the
actin cytoskeleton, thus allowing the transmission of biochemical and biophysical
(mechanical) signals [10].
11.2.4 GPIIb/IIIa Inside-Out Signaling
The activation of GPIIb/IIIa by inside-out signalling can be initiated by the stimulation of G protein coupled receptors on platelets after stimulation by soluble agonists
such as ADP and thrombin or by signals generated by platelet adhesion receptors
such as GPIb-IX-V or GPVI after binding to their respective ligands [11, 12]
(Fig.11.2). A common feature after agonist stimulation is the activation of phospholipase C (PLC)—liberating 1, 4, 5-inositol triphosphate (IP3) and diacylglycerol
(DAG), and subsequent release of intracellular calcium stores and activation of protein kinase C (PKC) and CalDAG-GEFI [13, 14]. PKC and CalDAG-GEFI are critical messengers that activate the small GTP binding protein Rap1b that is essential
for activation of GPIIb/IIIa [13, 14]. The importance of these signalling processes is
underscored by the observation that CalDAG-GEFI decient platelets demonstrate
impaired GPIIb/IIIa activation, prolonged bleeding times and protection from
thrombosis [13]. Likewise, CalDEF-GEFI deciency leads to impaired aggregation
responses to ADP and TxA2 [13]. However, some of these platelet function defects
seen in CalDEF-GEFI platelets can be overcome by more potent platelet agonists
such as collagen or thrombin [13]. These ndings have led to the notion that
CalDEF-GEFI mediates the rapid but reversible activation of Rap1b whilst PKC is
required for sustained Rap1b and therefore sustained GPIIb/IIIa activation.
The effector molecule of Rap1b is the small GTPase RIAM (Rap1-GTPinteracting molecule), which binds to Talin and localises it to the plasma membrane
[15]. Talin is a cytoplasmic partner of GPIIb/IIIa that serves as an essential mediator
of GPIIb/IIIa activation [16]. Talin exists in an autoinhibited basal state, however,
upon PKC mediated-RIAM activation, it becomes activated and serves to activate
GPIIb/IIIa [17]. In the resting, low afnity state, the transmembrane domains of the
and β3 subunits are closely associated and reinforced by a salt bridge between
α
IIb
these subunits which act to lock GPIIb/IIIa in the low afnity conformation [18].
Activated Talin binds to the cytoplasmic domains of the α
and β3 subunits and
IIb
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J. McFadyen and K. Peter
facilitates disruption of the salt bridge and the close association of the subunits [16].
This allows the extension of the β3 subunit and activation of GPIIb/IIIa to an
extended, high afnity conformation.
Another family of proteins that bind to the cytoplasmic domains of the β3 subunits are the Kindlins [19]. The Kindlins appear to play an important role in modulating Talin-mediated integrin activation, however the precise mechanisms
underpinning this function remain to be elucidated [19]. The fundamental roles of
RIAM, Talin-1 and Kindlin-3 are highlighted by the nding that mice with deciencies of these proteins exhibit defects in GPIIb/IIIa activation [15, 19].
11.2.5 Glycoprotein IIb/IIIa Outside-In Signalling
The binding of brinogen to GPIIb/IIIa results in integrin clustering and the formation of nascent multiprotein signalling complexes, followed by the formation of
larger, actin based signalling complexes [20, 21]. Whilst GPIIb/IIIa lacks intrinsic
signalling activity, the cytoplasmic tail of the β3 subunit contains evolutionary conserved NPXY motifs (arginine, prolene, X=any amino acid, tyrosine) that serve as
a recognition site for phosphotyrosine binding (PTB) proteins, which play integral
roles in propagating outside-in signals. Indeed, many of the over 20 identied cytoplasmic tail binding partners of GPIIb/IIIa are protein kinases or phosphatases [9].
As such, GPIIb/IIIa can transmit signals and contractile mechanical forces from the
extracellular to the intracellular environment. This is perhaps best reected by
GPIIb/IIIa ligation-dependent morphological changes that are associated with
dynamic alterations of the actin cytoskeleton [22]. These changes are mediated by
the effector molecules of the Rho GTPases such as cdc42, Rac1 and RhoA and
result in the extension of lopodia, lamellipodia and full platelet spreading. Thus,
GPIIb/IIIa outside-in signalling plays a central role in sustained platelet activation,
full platelet spreading, granule secretion and brin clot retraction [23].
Integrin outside-in signalling is initiated by ligand binding and clustering of
GPIIb/IIIa [20]. Src kinase, which is constitutively bound to the β
is then activated and recruits Syk [21]. Src and Syk may then phosphorylate multiple substrates, including the adapter proteins SLP-76 and ADAP, and PLCɣ2 that
promote GPIIb/IIIa association with the actin cytoskeleton and platelet activation
[24, 25]. Multiple other proteins that mediate outside-in signalling are recruited to
these nascent signalling complexes, including PI3 kinase (PI3K), Rac and VASP
(vasodilator-stimulated phosphoprotein) [11]. Whilst all the components of the signalling complex are yet to be fully elucidated, the β3 tail, PLCγ and α-actinin appear
to be three central proteins mediating the transition of the nascent signalling complex to an actin-based complex [26].
The activation and phosphorylation of Src kinases downstream of GPIIb/IIIa
ligation also induces the phosphorylation of focal adhesion kinase (FAK) [27]. FAK
is a ubiquitously expressed tyrosine kinase, which is phosphorylated and recruited
into the formation of the nascent focal adhesion complex [28]. In addition to
cytoplasmic tail,
3
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phosphorylated FAK, the nascent adhesion complex comprises of paxilin, talin, vinculin and actin [28, 29]. The assembled signalling complexes provide a means to
link the intracellular cytoskeleton to the extracellular GPIIb/IIIa. One of the major
downstream effectors of FAK is phosphorylated c-CBL, which associates and activates PI 3-kinase p110β (PI3Kβ) by recruiting the p85 regulatory subunit [30]. The
activation of PI3Kβ results in the stimulation of the small GTPase Rap1b, which is
critical for integrin dependent platelet adhesion and spreading on brinogen [30].
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11.2.6 Glycoprotein Ib-IX-V Complex
The Glycoprotein Ib-IX-V complex (GPIb-IX-V) is the second most abundant
platelet adhesion receptor with approximately 25,000 copies expressed on the platelet surface [31]. GPIb-IX-V is expressed exclusively on megakaryocytes and platelets and plays a critical role in haemostasis and thrombosis. This is highlighted by
the bleeding diathesis seen in patients with Bernard Soullier Syndrome, which is
caused by a congenital deciency in GPIb-IX-V expression or function [32]. The
GPIb-IX-V complex is composed of four different subunits—GPIbα, GPIbβ, GPIX
and GPV [33]. Each subunit is a type 1 transmembrane protein and contains a leucine rich repeat (LRR) domain in the extracellular domain [34]. The GPIb-IX complex is highly stable, and contains the GPIbα, GPIbβ and GPIX subunits in a ratio
of 1:2:1. The interactions of the subunits are critically important in stabilising the
individual domains and preventing their unwanted, premature proteolytic degradation. The importance of the interactions between the GPIbα, GPIbβ and GPIX subunits is highlighted by the fact that a mutation in one of these three subunits can
eliminate the GPIb-IX expression in platelets. In contrast to the tight association of
the GPIb-IX complex, it is thought the GPV subunit is only weakly associated via
the transmembrane domain and is not essential for the normal expression of the
GPIb-IX complex.
The extracellular N terminal region of the GPIbα subunit is the major ligand
binding domain of the GPIb-IX-V complex containing binding sites for ligands
such as vWF, thrombin, high molecular weight kininogen (HMWK), Factor XI and
XII, Mac-1 and P-selectin [35–37]. As a consequence, the GPIb-IX-V complex
plays an important role in mediating the adhesion of platelets to the subendothelial
matrix, the endothelium, leukocytes and facilitating the assembly of coagulation
factors on activated platelets [33].
The primary function of the GPIb-IX-V complex is to serve as an adhesion
receptor for vWF under high shear [
effects of shear stress on the GPIb-IX-V-vWF interaction may be biphasic, with
increasing force enhancing the bond strength (catch bonds) but above a force threshold, this interaction may weaken (slip bond) [39]. In contrast to integrin GPIIb/IIIa,
the interaction between GPIb-IX-V and vWF does not support stable platelet adhesion or the formation of stable platelet aggregates in isolation. Rather, the interaction between A1 domain of vWF and GPIb-IX-V has a fast on-off rate and as such
38]. Indeed, recent evidence suggests that the
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facilitates platelet rolling or translocation under high shear, thus facilitating rm
platelet adhesion mediated by the interactions of GPIIb/IIIa and/or GPVI with their
respective ligands [40]. Indeed, at high or pathological shear rates, GPIb-IX-V is the
only receptor that can mediate platelet adhesion and is therefore a prerequisite for
the capture of platelets under high shear conditions [41].
11.2.7 Glycoprotein VI
Glycoprotein VI (GPVI) represents a member of the immunoglobulin (Ig) superfamily of receptors and serves as the major receptor for collagen on platelets [42].
GPVI is expressed exclusively on platelets and megakaryocytes. GPVI is composed
of two extracellular immunoglobulin domains with a relatively short cytoplasmic
domain containing a basic amino acid rich region that can bind calmodulin and a
proline rich motif that can bind the Src homology 3 (SH3) domain of the SFKs, Fyn
and Lyn [43]. GPVI is complexed with the Fc receptor (FcR) γ chain via interactions
between the transmembrane and cytoplasmic domains [44]. Each FcRγ chain can
participate in signalling events via its tyrosine based activation motif (ITAM) [45].
The majority of GPVI exists as a monomer on the surface of resting platelets which
has a low afnity for collagen [46]. Platelet activation leads to the dimerisation of
GPVI, which has a unique conformation and enhanced afnity for collagen [46].
The major ligand of GPVI is collagen where collagen binding to GPVI produces
robust intracellular signals that play important roles in mediating thrombosis and
haemostasis. This is underscored by the recent description of patients with GPVI
deciency associated with a mild bleeding diathesis [47]. In response to vascular
injury and exposure of subendothelial collagen, platelets tether to vWF bound collagen, and then adhere and activate via GPVI interactions with exposed collagen.
The interaction of GPVI and collagen generate intracellular signalling events that
facilitate the activation of integrin α
2β1
and α
tering thus supporting stable platelet adhesion and activation [12, 48]. Whilst integrin α2β1 can also bind collagen, at arteriolar shear rates, GPVI plays the predominant
role in mediating stable platelet adhesion and aggregation [12].
, resulting in additional GPVI clus-
IIbβ3
11.2.8 Other Platelet Receptors
Recently, platelets have been demonstrated to express C-type lectin-like receptor
(CLEC-2), which binds the transmembrane glycoprotein, podoplanin. Podoplanin
is abundantly expressed in lymphatic endothelial cells, with the binding of podoplanin to platelet CLEC-2 triggering platelet activation. This interaction appears to
play an important role in mediating the development of lymphatic vessels, the integrity of the blood-lymphatic junction and maintaining vascular integrity in the context of inammation [49].
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Platelets also express the ‘scavenger’ receptor CD36 and a number of pattern recognition receptors such as the toll like receptors (TLRs). CD36 has been demonstrated to play an important role in mediating platelet activation in response to oxidised
low-density lipoprotein (oxLDL) and microparticles, whilst platelet TLR4 causes
platelet activation in response to the danger associated molecular patterns (DAMPs)
such as lipopolysaccharide (LPS) and high mobility group box 1 (HMGB1) [50, 51].
11.2.9 Platelet Granules
Platelets contain granules, which contain an array of proteins and other factors that
reinforce the haemostatic process but are also pro-inammatory and thereby contribute substantially to the inammatory role of platelets. Alpha granules are the
most abundant and contain platelet membrane proteins such as GPIIb/IIIa, glycoprotein (GP) Ib-IX-V in addition to the platelet adhesion proteins P-selectin, brinogen, vWF and coagulation Factor V [52, 53]. They also contain a vast array of
chemokines, cytokine and growth factors such as Chemokine Ligand 4 (CXCL4)
(PF4) and CXCL7 and the growth factors vascular endothelial growth factor (VEGF)
and platelet derived growth factor (PDGF) [54]. In contrast, dense granules contain
small molecules such as adenosine diphosphate (ADP), adenosine triphosphate
(ATP), calcium, serotonin and polyphosphate.
11.2.10 Platelet Membrane
The plasma membrane of resting platelets is similar to all eukaryotic cells in that the
phospholipids are distributed asymmetrically [55]. The outer leaet of the membrane is composed almost entirely of choline phospholipids whilst the inner leaet
contains all of the phosphatidylserine (PS) and the phosphoinositides [56]. The
asymmetrical distribution of phospholipids plays an important role in mediating
platelet activation and platelet procoagulant function [57, 58]. After platelet stimulation, the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P
hydrolysed to inositol triphosphate (IP3) and diacylglycerol (DAG) by the action of
phospholipase C (PLC) [59]. Both IP3 and DAG play essential roles in mediating
downstream effects that underpin platelet activation [60]. IP3 acts as a critical regulator of the cytosolic calcium concentration by acting as a secondary messenger that
facilitates the release of calcium from the endoplasmic reticulum [61]. DAG facilitates the translocation of protein kinase C (PKC) from the cytosol to plasma membrane [62]. Further, metabolism of phospholipid or DAG on the inner leaet by
phospholipase A
(PLA2) plays a fundamental role in arachidonic acid liberation
2
and subsequent thromboxane A2 (TxA2) synthesis [63].
The platelet membrane also plays a central role in mediating coagulation in vivo
by the provision of a phosphatidylserine (PS)-positive surface, which provides the
), is
2
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requisite negatively charged surface for the assembly of complexes of coagulation
factors [64]. After potent agonist activation, platelets can express PS on their outer
surface [65]. The importance of platelet PS exposure to haemostasis is highlighted
clinically by Scott syndrome, which is associated with impaired platelet PS exposure, manifesting as a severe bleeding diathesis [66].
11.3 Mediators ofPlatelet Activation
11.3.1 Soluble Agonists andTheir G Protein-Coupled
Receptors (GPCRs)
The release and generation of soluble agonists, such as ADP, thrombin and TxA2
from platelets and damaged cells play a critical role in the haemostatic and thrombotic response. These agonists activate platelets via the interaction with G protein
coupled receptors (Fig.11.1).
11.3.1.1 Adenosine Diphosphate (ADP) andtheP2Y Receptors
ADP is stored by platelets in dense granules at high concentrations and is released
upon platelet activation and degranulation where it activates platelets via autocrine
and paracrine signalling. Platelets have two ADP receptors—P2Y1 and P2Y12—
with both being important for normal platelet responses to ADP [67]. Indeed, P2Y1
decient mouse platelets demonstrate impaired platelet aggregation and shape
change in response to ADP [68]. Whilst P2Y12 decient mouse platelets show
impaired platelet aggregation, they do have a normal shape change in response to
ADP [69]. The importance of the P2Y receptors in facilitating paracrine platelet
signalling is demonstrated by the fact that platelet activation is impaired in response
to low dose TxA
and thrombin in the absence of ADP receptors [70].
2
The P2Y1 receptor is coupled to the Gq subunit, which upon activation, leads to
the hydrolysis of PI(4,5)P2 by PLCβ and therefore intracellular calcium mobilisation and PKC activation—both of which are important for integrin activation and
granule secretion [71, 72]. P2Y1 Gq mediated calcium increase has been conrmed
to be critical for ADP-induced platelet shape change via RhoA activation [72]. The
P2Y12 receptor is linked to Gi, and mediates PI3K activation, thus playing a central
role in the activation of the small GTPase, Rap1b which is an essential mediator of
sustained integrin activation [73]. Further, Gi signalling inhibits the production of
cyclic AMP (cAMP) thus relieving the platelet inhibitory effect of cAMP dependent
protein kinase [74]. The importance of P2Y
and P2Y12 in platelet haemostatic func-
1
tion is highlighted by the severe bleeding phenotype in P2Y1 and P2Y12 decient
mice [75]. The P2Y12 receptor is irreversibly inhibited by the thienopyridine,
clopdiogrel.
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11.3.1.2 Thromboxane A2 (TxA2) andtheThromboxane Receptor (TP)
TxA2 is generated in platelets by the conversion of arachidonic acid to endoperoxidases by cyclo-oxygenase and their subsequent metabolism to TxA2 by thromboxane
synthase [76]. The effect of TxA2 as a paracrine messenger to amplify platelet activation is spatially constrained due to its short half-life. The receptor for TxA2, TP, is
coupled to Gq and G13 and as such, plays a role in platelet degranulation, shape change,
integrin activation and thus aggregation [72, 77]. The role of TxA2 and the TP receptor in platelet function is demonstrated by TP decient mice, which demonstrate a
bleeding phenotype and the inability to form stable thrombi in vivo [78]. Clinically,
the anti-platelet drug aspirin inhibits cyclo-oxygenase and thus TxA2 generation.
11.3.1.3 Thrombin andtheProtease Activated Receptors (PAR)
Thrombin is the central effector protease of the coagulation cascade in addition to
being the most potent platelet agonist [72]. Thrombin is generated on the surface of
activated platelets and endothelial cells and can activate platelets by interactions with
thrombin receptors on the platelet surface. Human platelets express two thrombin
receptors, the protease activated receptors PAR1 and PAR4, which are linked to Gq
and G13 proteins [79, 80]. PAR1 is the major thrombin receptor on human platelets
as it can mediate platelet activation at low thrombin concentrations whereas PAR4
has a much lower afnity for thrombin and thus only activates platelets in the presence of high thrombin concentrations [79]. The activation of PAR1 by thrombin is
rather unique in that thrombin cleaves an N-terminal extracellular domain of PARs,
which creates a tethered ligand that activates the receptor [79]. PAR1 and PAR4 are
coupled to Gq and G13 intracellular signals [81]. PAR1 and PAR4 ligation triggers
potent Gq signalling and therefore results in the activation of PLCɣ and resultant
activation of PKC and calcium mobilisation, required for integrin activation and
platelet degranulation [82]. The activation of G
results in Rho and Rho kinase acti-
13
vation, which via inhibition of myosin light chain phosphatase, leads to actin contraction and rapid shape change and degranulation [83]. Whilst there has been some
debate as to whether PAR receptors directly couple to Gi, it appears that thrombin
does not directly induce Gi mediated signals. Rather, Gi signalling is induced by
thrombin indirectly via released ADP and subsequent P2Y12 signalling [84].
11.4 Platelet Thrombus Formation
11.4.1 Platelet Adhesion
The inciting event underlying pathological thrombus formation is typically the erosion or rupture of an atherosclerotic plaque [85]. This leads to the exposure of a number of proteins and vascular matrix components that act as adhesive ligands for
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Fig. 11.3 The prothrombotic function of platelets. The inciting event leading to the formation of
a pathological arterial thrombus is the rupture of an atherosclerotic plaque. This leads to the exposure of a number of highly reactive subendothelial matrix proteins such as vWF and collagen.
Under arterial shear, platelets tether to the site of arterial injury via interactions of platelet GPIbvWF. These interactions facilitate the engagement of other receptor-ligand interactions such as
GPVI-collagen that lead to platelet activation, ultimately leading to activation of GPIIb/IIIa
(inside-out signalling). Activation of GPIIb/IIIa allows stable platelet-platelet interactions (platelet
aggregation) and the formation of a platelet thrombus. The release of soluble agonists within the
connes of the nascent platelet thrombus amplies platelet activation and thrombin generated
catalyses brinogen to brin thus forming a mesh that stabilises the platelet rich thrombus
J. McFadyen and K. Peter
platelets and also platelet activators. Indeed, the subendothelial matrix is signicantly
altered with the development of an atherosclerotic plaque such that collagen, brinogen/brin and tissue factor are all highly expressed. Upon plaque rupture, the initial
capture and adhesion of platelets to the vessel wall under arterial shear is mediated by
platelet GPIb-vWF interactions. Here, vWF, derived from both the injured endothelium and circulating plasma pool, binds via its A3 domain to collagen thereby allowing the capture of platelets via interaction between GPIb and the vWF A1 domain
[86]. The GPIb-vWF interaction is characterised by a fast ‘on’ and ‘off’ rate and
therefore whilst it can support the initial tethering platelets, other interactions are
required to mediate stable platelet adhesion. In this regard, by slowing down translocating platelets, GPIb-vWF interactions facilitate the binding of GPVI to collagen and
GPIIb/IIIa to brinogen, which then mediate stable platelet adhesion [86] (Fig.11.3).
11.4.2 Platelet Activation andAggregation
Platelets are either activated by soluble agonists or by receptor-ligand interactions
induced by platelet adhesion, both generating downstream signalling pathways ultimately leading to autocrine and paracrine platelet activation/recruitment. Upon
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