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(Circulation Research. 2007;100:1673.)
© 2007 American Heart Association, Inc.

Reviews

Adhesion Mechanisms in Platelet Function

Zaverio M. Ruggeri, G. Loredana Mendolicchio

From The Roon Research Center for Arteriosclerosis and Thrombosis, Division of Blood Cell and Vascular Biology, Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, Calif. Present address for G.L.M.: Istituto Clinico Humanitas, Rozzano, Milan, Italy.

Correspondence to Zaverio M. Ruggeri, MD, The Scripps Research Institute, MEM-175, 10550 N Torrey Pines Rd, La Jolla, CA 92037. E-mail ruggeri{at}scripps.edu

This Review is part of a thematic series on Mechanisms, Models, and In Vivo Imaging of Thrombus Formation, which includes the following articles:

Activation of Platelet Function Through G Protein–Coupled Receptors

Platelets As Immune Cells: Bridging Inflammation and Cardiovascular Disease

In Vivo Thrombus Formation in Murine Models

Clinical Aspects of Platelet Inhibitors and Thrombus Formation

Adhesion Mechanisms in Platelet Function
Bernhard Nieswandt and Ulrich Walter Guest Editors

	Abstract

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Platelet adhesion is an essential function in response to vascular injury and is generally viewed as the first step during which single platelets bind through specific membrane receptors to cellular and extracellular matrix constituents of the vessel wall and tissues. This response initiates thrombus formation that arrests hemorrhage and permits wound healing. Pathological conditions that cause vascular alterations and blood flow disturbances may turn this beneficial process into a disease mechanism that results in arterial occlusion, most frequently in atherosclerotic vessels of the heart and brain. Besides their relevant role in hemostasis and thrombosis, platelet adhesive properties are central to a variety of pathophysiological processes that extend from inflammation to immune-mediated host defense and pathogenic mechanisms as well as cancer metastasis. All of these activities depend on the ability of platelets to circulate in blood as sentinels of vascular integrity, adhere where alterations are detected, and signal the abnormality to other platelets and blood cells. In this respect, therefore, platelet adhesion to vascular wall structures, to one another (aggregation), or to other blood cells, represent different aspects of the same fundamental biological process. Detailed studies by many investigators over the past several years have been aimed to dissect the complexity of these functions, and the results obtained now permit an attempt to integrate all the available information into a picture that highlights the balanced diversity and synergy of distinct platelet adhesive interactions.

Key Words: adhesion molecules • platelets • vascular biology • extracellular matrix • collagen

	Introduction

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Platelets in mammals are anucleated cells that originate from the cytoplasm of bone marrow megakaryocytes¹ and circulate in blood as sentinels of vascular integrity. They show no interaction with the inner surface of normal vessels but adhere promptly where endothelial cells are altered or extracellular matrix substrates are exposed.² This is a critical initial step in hemostasis and thrombosis, as well as in inflammatory^3,4 and immunopathogenic responses.⁵ The functions of mammalian platelets are conserved throughout evolution and closely reflect those of nucleated thrombocytes in all other vertebrates.^6–9 After adhering to vascular lesions, platelets can rapidly recruit to the site of injury additional platelets, which are necessary to achieve hemostasis, or different types of leukocytes, which set off host defense responses. Such selective recruitment is orchestrated by activation pathways stimulated by the initial adhesive interactions and by soluble agonists released or generated locally, which lead to the appearance on the platelet membrane of different adhesive molecules capable of attracting distinct circulating cells. Platelet adhesion in a broad sense is the study of the adhesive properties of platelets. In this review, we focus on the mechanisms responsible for the initial interaction of platelets with thrombogenic surfaces and the subsequent events that lead to thrombus propagation and stabilization mostly through interplatelet contacts induced by cellular activation. We also discuss some key aspects of platelet adhesive functions that are relevant to inflammation, atherogenesis, and host defense mechanisms (see the online data supplement, available at http://circres.ahajournals.org).

	Platelet Adhesion and Platelet Aggregation: Two Sides of the Same Coin

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Adhesion and aggregation are often considered as distinct processes through which platelets establish individual contacts with extracellular surfaces or stick to one another, forming clumps, respectively. Yet, this distinction is based more on the interpretation of experimental studies than on cogent mechanistic reasons, because both adhesion and aggregation involve the transition of platelets from free flow in blood to arrest onto a surface, often mediated by the same adhesive ligand and receptor pairs. In the case of platelet adhesion, the surface is the extracellular matrix or the membrane of other cells of the vessel wall and surrounding tissues, and the adhesive substrates are endogenous matrix or cell membrane proteins and proteoglycans, along with locally bound selected plasma components. In the case of platelet aggregation, the surface is the membrane of other platelets that have already become immobilized at sites of thrombus formation and present membrane-bound adhesive substrates, either following activation-induced translocation from internal storage compartments or bound from plasma. An important consideration is that, by virtue of the mechanistic similarities, the same fluid dynamic constraints influence both platelet adhesion and aggregation.

	Effects of Hydrodynamic Conditions on Platelet Adhesion and Aggregation

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Platelets are essential for normal hemostasis and, in particular, for arresting bleeding from arterioles, where shear stress is elevated (see the online data supplement).¹⁰ In pathological conditions, platelets are a major contributor to arterial thrombosis, which typically occurs at sites of atherosclerosis with stenosis of the vessel lumen, where shear-stress values are considerably higher than in the normal circulation.^11–13 For these reasons, and because the complex events that regulate platelet function are influenced by the flow of blood, the mechanisms that support platelet adhesion and aggregation under the constraints of elevated shear stress are of particular interest in the study of thrombus formation. Shear rate and shear stress have different effects on cellular adhesive interactions. The shear rate is directly related to flow velocity, including the velocity of cells in the fluid layer adjacent to the vessel wall, and limits the time of contact between membrane receptors and immobilized substrates on the vessel wall, thus the on-rate of the adhesive interaction. As a consequence, the efficiency of cell recruitment onto the surface decreases with increasing shear rate. Shear stress, in contrast, influences the lifetime of an adhesive bond once formed, thus the off-rate of the interaction, and the consequence is decreased efficiency caused by detachment of adherent cells with increasing shear stress. The point has been well documented for leukocyte adhesion.¹⁴

Different pathways of platelet adhesion are variably affected by increasing shear force depending on the biomechanical properties of each receptor–ligand pair. Above a threshold shear rate of 500 to 800 sec^–1 in human blood^15,16 and 2000 to 5000 sec^–1 in mouse blood,¹⁷ only the interaction between immobilized von Willebrand factor (VWF) A1 domain (VWF-A1) and membrane glycoprotein Ib (GPIb) has a sufficiently fast on-rate to initiate platelet adhesion.¹⁸ The higher threshold in the mouse is likely the consequence of a smaller platelet size. Thrombus development alters the hemodynamic conditions by restricting the lumen through which blood flows. To maintain the same volumetric flow rate (ie, the volume of blood transported per unit time) in spite of the restriction, the velocity of flow must increase, resulting in higher shear rate and stress. This explains why shear rates in excess of 20 000 to 40 000 sec^–1 develop at or just upstream of a severe stenosis in a human atherosclerotic coronary artery.^11–13 For the same reason, the shear rate on the membrane of immobilized platelets exposed to flowing blood at the surface of an arterial thrombus is progressively higher as the protrusion into the vessel lumen increases. Thus, the fluid dynamic constraints that influence the adhesion of single platelets to the vessel wall affect also the recruitment of circulating platelets into a growing thrombus, as demonstrated by the required function of GPIb and VWF-A1 to support platelet aggregation above a threshold shear rate (Figure 1). It is important to note that the threshold discussed here is not a minimum shear rate value to engage the function of immobilized VWF-A1, which can mediate platelet tethering even under venous slow flow conditions¹⁸; rather, it is an upper limit for the function of most other platelet adhesive bonds in the absence of VWF.

View larger version (58K): [in this window] [in a new window]   Figure 1. Role of GPIb in platelet aggregation during thrombus growth. Blood containing as anticoagulant the -thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone dihydrochloride (PPACK) (final concentration, 93 µmol/L) was perfused over collagen type I fibers for 100 seconds at the wall shear rate of 1500 sec–1. At this point, the height of thrombi was measured from confocal (z) sections, as shown in the stacks in A, while the perfusion continued for an additional 740 seconds with blood containing either buffer or function-blocking monoclonal antibodies directed against GPIb or IIbß3, or both, as indicated. The flow rate was decreased such that the calculated shear rate was 300 sec–1 at the collagen-coated surface but >1000 sec–1 at the surface of thrombi with a height >20 µm. After a total perfusion time of 840 seconds, thrombus height was measured by confocal sections, as shown in the stacks in B. The results demonstrate that the adhesive function of GPIb is as necessary as that of IIbß3 to sustain platelet aggregation at the edge of a growing thrombus. Modified from Ruggeri ZM et al130 and reprinted with permission.

	Initial Platelet Adhesion to Thrombogenic Surfaces

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Substrates and Receptors for Platelet Adhesion to the Vessel Wall
The hemostatic response to vascular injury is contingent on the nature of the lesion. Depending on the matrix proteins exposed to blood and the hemodynamic conditions, platelet adhesion requires the synergistic function of different platelet receptors, ultimately leading to platelet activation and aggregation. The extracellular matrix components that react with platelets include different types of collagen, VWF, fibronectin,¹⁹ and other adhesive proteins such as laminin,²⁰ fibulin,²¹ and thrombospondin.²² Fibrinogen/fibrin¹⁸ and vitronectin²³ are not synthesized by vascular wall cells but must be considered as potentially relevant thrombogenic substrates because they become immobilized onto extracellular matrix at sites of injury. One can assume that all tissue components capable of interacting with platelets can contribute to the initiation of thrombus formation when exposed to blood, even though only a few may have essential roles. Of note, experiments with purified proteins are useful to establish specific mechanisms of action but may not reflect functions within the complex supramolecular assembly of extracellular matrices. As a consequence, the relative contribution of several adhesive interactions to the process of platelet adhesion to vascular surfaces remains to be elucidated in detail. This is true in particular for insoluble proteins that exist only in the extracellular matrix, such as collagen, or for proteins that may exist in a soluble form in plasma, such as fibronectin, but differ in their structure and conformation when they assemble into complex matrices.^24,25

Platelet Adhesion to VWF
Subendothelial Matrix VWF and Immobilized Plasma-Derived VWF
Among the substrates required for normal thrombus formation, VWF is unique for its role in initiating platelet adhesion and sustaining platelet aggregation under conditions of elevated shear stress.^16,18 These functions are performed primarily through the tethering of GPIb in the platelet membrane GPIb-IX-V receptor complex²⁶ to the A1 domain of immobilized VWF exposed to flowing blood. As a constitutive component of the extracellular matrix of endothelial cells,²⁷ in which it is associated with collagen type VI filaments,^28–30 subendothelial VWF can directly support platelet adhesion.^31–35 Nonetheless, hemostasis can be normal in the absence of endogenous endothelial VWF if plasma VWF is present (see the online data supplement). Consequently, the interaction of circulating VWF with exposed vascular and perivascular tissues is a key early event in thrombus formation.

The Transition From Soluble to Immobilized VWF
Plasma VWF can become immobilized onto subendothelial surfaces through the binding to extracellular matrix components and through self-association with other VWF multimers. The main substrate capable of binding VWF is collagen,³⁶ particularly types I and III in deeper layers of the vessel wall and microfibrillar collagen type VI in the subendothelial matrix,^30,37–39 with heparin, sulfatides, and fibrin providing additional interaction sites (see the online data supplement). Two of the 3 type A domains in VWF, A1 and A3, can mediate binding to collagens, and their respective roles may vary depending on the type of collagen involved and the fluid dynamic conditions.^38,40 The ability to self-associate represents an additional mechanism for the transition from soluble to immobilized VWF, in which case circulating multimers interact in a reversible manner with matrix-bound and endogenous subendothelial VWF.⁴¹ This mechanism has been demonstrated by immobilizing a mutant VWF devoid of domain A1 (A1-VWF), thus unable to promote platelet adhesion onto collagen and showing that GPIb-mediated tethering was restored by the presence of soluble VWF in plasma (Figure 2). Very large VWF multimers locally released by stimulated endothelial cells⁴² may enhance the efficiency of the process, as these molecules form high-strength bonds with GPIb.⁴³ Self-association of VWF multimers can occur onto the platelet surface⁴⁴ under conditions of hydrodynamic shear that favor the binding of soluble VWF.⁴⁵ The self-association of VWF apparently involves multiple domains,⁴⁶ and none has been identified as essential, including A1 and A3.⁴¹ In summary, the available evidence suggests that different types of injury may elicit distinct pathways for the local immobilization of soluble VWF. As a consequence, for example, VWF binding to collagen may not be essential to ensure normal hemostasis but may be a primary determinant of the pathological thrombogenic response caused by the rupture of collagen-rich atherosclerotic plaques. From an experimental point of view, it is difficult to recreate such a functional diversity using purified molecules, which can explain some of the inconsistencies found in the literature with respect to the mechanisms of VWF binding to vascular surfaces (see the online data supplement).

View larger version (55K): [in this window] [in a new window]   Figure 2. Functional self-association of VWF multimers. A washed blood cells suspensions devoid of plasma proteins and containing EDTA to block integrin function and prostaglandin E1 to block platelet activation was perfused over immobilized collagen type I fibers at the wall shear rate of 1500 sec–1. A, Control experiment with normal multimeric VWF added to the cell suspension. The A3 domain mediates VWF binding to collagen, and the A1 domain interacts with platelet GPIb. Tethered platelets are seen rolling on the surface, which is represented by an electron micrograph of collagen fibrils. B and C, Experiments performed after adding to the cell suspension, respectively, recombinant VWF devoid of the A3 domain (A3-VWF), which cannot bind to collagen, or devoid of the A1 domain (A1-VWF), which binds to collagen but cannot interact with platelet GPIb. In either case, no platelets are seen tethered to the surface. D, Collagen was precoated with A1-VWF multimers, which cannot initiate platelet tethering, and then exposed to the blood cell suspension containing A3-VWF. Although the latter cannot bind directly to collagen (see b), it could compensate for the lack of A1 domain in the surface-bound VWF and restore platelet tethering. The association of VWF multimers with one another can explain this result; the 2-sided arrows between multimers indicate that the association is reversible. The images are single frames from a real-time recording representing an area of 65 536 µm2. The bar graph (E) shows the number of platelets tethered to the surface under the different experimental conditions described above (mean±SEM of 2 separate experiments). Adapted from Savage et al41 and reprinted with permission.

The Distinctive Functional Properties of Immobilized and Soluble VWF
Platelets have no measurable interaction with soluble VWF in the circulation but adhere promptly to exposed immobilized VWF. Such a tight regulation is necessary to prevent intravascular platelet aggregation and has led to the concept that surface-bound VWF must undergo a conformational change to make the interaction with GPIb possible and initiate platelet adhesion. Indeed, VWF molecules may change shape depending on hemodynamic conditions, so that on binding to the vessel wall under high shear stress, they may appear as elongated filaments rather than the loosely coiled structures seen under static or low-shear-stress conditions.⁴⁷ Such an "uncoiling" may expose the repeating functional sites present in multimeric VWF, allowing a more efficient support of adhesive interactions as a result of multivalent binding. Three-dimensional structural studies⁴⁸ have shown that more subtle conformational changes can occur in the GPIb-binding A1 domain as a result of amino acid substitutions, such as those causing type 2B von Willebrand disease,⁴⁹ which overcome the affinity barrier for soluble VWF binding to platelets. These results prove that conformational changes can regulate the interaction between VWF-A1 and GPIb, but there is no evidence that the transition from soluble to surface-immobilized VWF induces these or similar conformations. Studies with a specific antibody fragment,⁵⁰ a "nanobody," support the concept of a common "active" conformation in the VWF A1 domain of surface-bound multimers, soluble ultralarge multimers released by endothelial cells, and mutant type 2B plasma VWF, in contrast to the "inactive" conformation of normal plasma VWF. In fact, the nanobody appears to bind preferentially to the A1 domain of VWF species, with enhanced affinity for GPIb, indicating that they may share the same conformation. It remains to be determined whether such a conformation is dynamically transient or reflects one of the known crystallized structures.^48,51,52 A particularly relevant "active" form of soluble VWF is represented by the ultralarge multimers⁴³ released from the storage granules of stimulated endothelial cells and platelets.^53,54 Ultralarge VWF multimers function locally, but under normal conditions, they do not accumulate in circulating blood⁵⁵ because they are processed by a specific protease, ADAMTS-13⁵⁶ (see the online data supplement).

Membrane Receptors and the Mechanism of Platelet Tethering to VWF
Platelets have 2 main binding sites for VWF,^57,58 GPIb in the GPIb-IX-V complex²⁶ and the integrin _IIbß₃.⁵⁹ A second ß₃ integrin, _vß₃, albeit present at much lower density than _IIbß₃,⁶⁰ may contribute to the platelet binding of VWF through the ligand Arg-Gly-Asp (RGD) sequence, an interaction shown to occur on endothelial cells.⁶¹ Both VWF platelet receptors are promiscuous and bind several ligands that may mediate adhesion to other platelets and cells. In particular, the GPIb-IX-V complex is a counter receptor for P-selectin⁶² and for the leukocyte integrin Mac-1 (₂ß_M),⁶³ supporting 2 interactions that may contribute more to inflammatory responses⁴ than to platelet thrombus formation. The integrin _IIbß₃, on the other hand, binds several ligands, in addition to VWF, that are key to the process of platelet adhesion and aggregation, primarily fibrinogen,⁶⁴ fibronectin,⁶⁵ and CD40 ligand.⁶⁶

The distinguishing feature of the interaction between GPIb and VWF-A1 is the ability to support activation-independent platelet tethering to thrombogenic surfaces even when the velocity of blood is elevated. The interaction has a fast dissociation rate, exhibiting the characteristics of a selectin-like bond,⁶⁷ and the common paradigm is that it cannot support irreversible adhesion¹⁸; thus, platelets tethered to the vessel wall solely through VWF-GPIb binding move constantly in the direction of flow. In inflamed tissues, this function may support the initial platelet contact with stimulated endothelial cells,⁶⁸ a surface onto which membrane-bound VWF and P-selectin, which also mediates transient adhesion and rolling,⁶⁹ may be the only adhesive substrates. Initial transient interactions between platelets and reactive surfaces may be essential for allowing a modulated response while surveying vessel wall integrity, as commitment to irreversible adhesion after each initial contact could have adverse consequences, including tissue damage. The presence of additional structures signifying a serious lesion may be the required trigger for subsequent steps such as irreversible platelet adhesion and accumulation. The GPIb-mediated translocation velocity onto immobilized VWF is typically less than 2% of the free flow velocity of noninteracting platelets at the same distance from the luminal surface. This slow motion allows the establishment of additional bonds through receptors that belong mostly, but not necessarily, to the integrin superfamily. Such receptors, many of which depend on platelet activation to express function, typically have an intrinsically slower rate of bond formation but are capable of mediating stable interactions that lead to the definitive arrest of individual platelets and subsequent thrombus development. Notable in this regard is the role of the activated integrin _IIbß₃, which binds to the Arg-Gly-Asp sequence in VWF itself^57,58 or to other adhesive substrates in a complex matrix, and of collagen and its receptors.^16,70,71 When VWF is bound to collagen, the transition from rolling to stable adhesion occurs more rapidly than on immobilized VWF alone and thrombus development occurs at higher shear rates than on collagen without VWF.¹⁶ Such a consideration highlights the true synergistic function of the VWF–collagen complex, which also leads to multiple activating signals coupled, in part, to the VWF-GPIb interaction (see the online data supplement).

An Integrated View of VWF-Mediated Platelet Adhesion and Aggregation
The concept that the VWF-GPIb interaction cannot support long-lasting adhesion must be modified in view of the recently demonstrated ability of nonactivated platelets to form aggregates that attach firmly to immobilized VWF under extremely high shear-stress conditions (Figure 3).⁷² Several unique features characterize this mechanism of platelet adhesion to extracellular surfaces and to one another, marking substantial differences with the process of single-platelet rolling. Perhaps the most relevant distinction is that GPIb-mediated, long-lasting adhesion and aggregation only occurs above a threshold shear rate of 10 000 sec^–1, a feature that highlights its potential importance for pathological arterial thrombosis. A second key distinction is that platelet adhesion and aggregation at pathologically elevated shear rates depends on soluble as well as surface-bound VWF. Single-platelet adhesion and rolling, in contrast, requires only immobilized VWF, even though it is also enhanced by the presence of soluble VWF at the higher shear rates,⁷² likely as a result of VWF multimer self-association favored by shear-induced binding to platelets. A third and equally relevant feature is that GPIb-mediated and VWF-dependent firm platelet adhesion and aggregation occurs without any requirement for platelet activation and integrin function. Such a statement should not be taken to mean that activation has no influence on platelet thrombus formation at pathologically elevated shear rates, as it remains essential for the stability of aggregates and their attachment to the reactive surface. It is intuitive, however, that the ability of platelets to aggregate onto surfaces even before activation takes place greatly favors the establishment of growing thrombi in a high-shear-rate environment, in which elevated tensile stress limits the efficiency of adhesive bonds and rapid flow reduces the concentration of agonists required for activation. Under challenging hydrodynamic conditions, therefore, platelet interactions with adhesive surfaces and with one another appear to be synergistic. Of note, ADAMTS-13 can cleave circulating VWF multimers while they mediate activation-independent interplatelet cohesion under high shear stress, thus dispersing the aggregates.⁷³

View larger version (68K): [in this window] [in a new window]   Figure 3. Activation-independent platelet adhesion and aggregation at the interface of immobilized and soluble VWF. A, Blood containing 93 µmol/L PPACK as anticoagulant, the fluorescent dye mepacrine (10 µmol/L), prostaglandin (PG)E1 (10 µmol/L) to inhibit platelet activation, and EDTA (5 mmol/L) to prevent ligand binding to integrins, was perfused over immobilized VWF (20 µg/mL coating concentration). The white line delimits VWF-coated (to the left) from uncoated glass. Single platelets adhere when the shear rate is 3000 sec–1 (top); rolling aggregates (some identified by arrowheads) form at 20 000 sec–1 (bottom). B, Perfusion over immobilized VWF of washed blood cells suspended in buffer (20 mmol/L Hepes, 150 mmol/L NaCl, pH 7.4). In the absence of soluble VWF, single platelets adhere when the shear rate is 3000 sec–1 (upper left), and fewer single platelets adhere at 24 000 sec–1 (upper right). After adding soluble VWF (20 µg/mL), single platelets adhere at 3000 sec–1 (lower left; an arrow points to a single platelet shown for reference), but aggregates form at 24 000 sec–1 (lower right; arrows point to a rolling aggregate and an inset highlights a stretched aggregate during stationary adhesion). C, Perfusion over immobilized VWF of washed blood cells with added soluble VWF and anti-VWF A1 domain monoclonal antibody (NMC-4, 20 µg/mL).133 No platelet adhesion is detected. Reprinted from Ruggeri et al72 with permission.

Platelet Adhesion to Collagen
Different types of collagen (particularly I, III, and VI) are thought to be among the most active vessel wall components in the initiation of platelet adhesion and aggregation,³⁶ but the relevance of their contribution to hemostasis and thrombosis relative to other extracellular matrix molecules remains to be defined. The conformation of different collagens may influence the mechanisms of interaction with platelets under flow and subsequent activation, which is a relevant factor in the interpretation of experimental results.⁷⁴ Acid-insoluble fibers display a characteristic banded pattern attributable to the regular staggering of collagen monomers,^36,75 whereas pepsin-solubilized microfibrils lack this pattern and form spiral structures (Figure 4).⁷⁴ Spiraled collagen formed by fibrils in helical assembly has been found in both normal and pathological vascular tissues, and may result from the degradation of fibers by matrix metalloproteinases, particularly matrix metalloproteinase 1.⁷⁶ When immobilized, all of these substrates are thrombogenic but elicit platelet adhesion and aggregation through different mechanisms. This is best illustrated by the distinct consequences of blocking one of the platelet collagen receptors, the integrin ₂ß₁, which impairs thrombus formation on spiral microfibrils but not on banded collagen fibers.⁷⁴ In contrast, blocking the other receptor, GPVI, impairs thrombus formation on all types of collagen fibers.^77,78 In fact, there appears to be a requirement for GPVI and ₂ß₁ cooperation only when collagen fibers, and presumably their triple helical configuration, are not intact (Figure 4).^74,77 As collagen is an insoluble matrix protein, preparations used in ex vivo experimental studies are often treated with proteases, such as pepsin, for solubilization, with the consequence of a functional behavior distinct from that of the native protein. For example, pepsin-treated acid soluble collagen contains spiraled fibrils that can lead to overestimate the role of ₂ß₁ in the interaction with platelets.

View larger version (41K): [in this window] [in a new window]   Figure 4. Schematic representation of the mechanisms of platelet adhesion to collagen. The collagen receptors, 2ß1 and GPVI, are depicted along with the GPIb-IX-V complex. Collagen and VWF are represented by electron micrographs (not in scale). Two forms of collagen type I are shown: "native" acid-insoluble fibers (on the right) and pepsin-treated acid-soluble "spiraled" fibrils (on the left), with a VWF multimer shown as if adhering to the native collagen fiber. On a surface in which collagen and VWF form a functional unit, initial platelet tethering is mediated primarily by GPIb binding to VWF-A1 under high-flow conditions, otherwise by GPVI binding to collagen, specifically to the prototypic repeating sequence Gly-Pro-Hyp (where Hyp is hydroxyproline) typical of the fibril-forming collagens. Both receptors can signal, but the role of GPVI is predominant in this respect. The integrin 2ß1 requires activation by an inside-out signal for high-affinity binding to collagen; however, on partially degraded "spiraled" fibrils, it may be essential for the initial tethering through recognition of the sequence Gly-Phe-Hyp-Gly-Glu-Arg. The fact that GPVI is not sufficient for adhesion to this substrate attests to the conformational disruption of its recognition site in partially degraded collagen.

The Platelet Collagen Receptors
The platelet membrane is endowed with several collagen receptors, including the integrin ₂ß₁,^79–81 GPVI,^82,83 GPIV (CD-36),^84–86 and the 65-kDa protein (p65) reportedly specific for type I collagen.⁸⁷ GPIV, which may be required for the interaction with collagen type V⁸⁸ and is associated with Src kinases,⁸⁹ is unlikely to make a major contribution to platelet activation because it is absent in approximately 5% of the Japanese population without hemostatic impairment. Likewise, the relevance of p65, which may be linked to nitric oxide generation,⁹⁰ is unproven. Of the proposed receptors, only ₂ß₁ and GPVI have a defined role in platelet–collagen interactions, but, in spite of extensive studies, their relative importance with respect to both adhesion and activation remains a topic for debate (Figure 4). Problems associated with the isolation of collagen from extracellular matrices, as discussed above, and possible but poorly understood differences between human ex vivo and mouse in vivo experimental systems explain the lingering uncertainties.^36,71

The Integrin ₂ß₁
The integrin ₂ß₁ corresponds to the platelet membrane GPIa-IIa, originally identified as the very-late-activation antigen-2 on T cells⁹¹ and class II extracellular matrix receptor on fibroblasts.⁸⁰ The expression of ₂ß₁ on normal platelets varies by as much as 1 order of magnitude,⁹² and may positively correlate to the rapidity of the initial phase of platelet adhesion to collagen.⁹³ Several collagen sequences have been identified as targets for ₂ß₁ binding,³⁶ and the most relevant appears to be Gly-Phe-Hyp-Gly-Glu-Arg (where Hyp is hydroxyproline), which has been crystallized in complex with the receptor.⁹⁴ Like other integrins, ₂ß₁ requires activation and divalent cations to engage its ligands with high affinity, and although this may be a requisite for signaling (see the online data supplement), it may not be necessary for the initial contact. Thus, even in a low affinity state, ₂ß₁ may be capable of mediating platelet adhesion to collagen preceding GPVI-induced activation,³⁶ like nonactivated _IIbß₃ mediates adhesion to immobilized fibrinogen.⁹⁵ Abnormalities of ex vivo platelet interactions with collagen have been reported in ₂-deficient mice, ranging from relatively mild, based on evaluating platelet adhesion under static conditions,⁹⁶ to severe, based on evaluating platelet adhesion under flow conditions.⁹⁷ In the latter study, with blood perfused over fibrillar collagen type I at shear rates between 400 and 1300 sec^–1, the isolated deficiency of ₂ß₁ or GPVI-Fc receptor -chain (FcR-) complex caused an equivalent partial defect of platelet thrombus formation, whereas the combined deficiency caused complete inhibition.⁹⁷ The conclusion was that the 2 receptors perform independent and critical roles in the interaction with collagen and operate synergistically for optimal function. Such a concept is in contrast to the alternative opinion that GPVI can mediate all aspects of collagen-dependent platelet adhesion and activation, even in the absence of ₂ß₁.⁷¹ Although experimental conditions may tip the balance in favor of one of the other view, the 2 receptors are likely to contribute variably to platelet thrombus formation depending on the nature of the vascular lesion and the presence of yet unknown factors that can influence platelet and extracellular matrix components. In this regard, it is noteworthy that the congenital deficiency of either ₂ß₁^81,98 or GPVI⁸² in humans results in a mild bleeding diathesis.

Experiments using in vivo mouse models have led to conflicting results on the role of ₂ß₁ in platelet function. Some authors, who investigated the consequences of a mechanical injury in the mouse right common carotid artery, concluded that multiple integrin–ligand interactions synergize in promoting initial platelet adhesion (the study did not address thrombus formation) to the lesion, but without a significant contribution by ₂ß₁. Rather, in agreement with previous ex vivo experiments using human blood and inhibitory antibodies,¹⁶ the results pointed to an important role of other platelet ß₁ integrins (₅ß₁ and ₆ß₁; see below, under Platelet Adhesion to Fibronectin and under Platelet Adhesion to Laminin) in addition to the expected relevant contribution by _IIbß₃.⁹⁹ In contrast, other investigators who used a photochemical injury always in the mouse right common carotid artery, concluded that ₂ß₁ plays a critical role in vascular thrombosis, although it is seemingly not involved in the formation of intravascular thrombi and pulmonary emboli following an injection of collagen.¹⁰⁰ These authors observed that thrombus formation in the ₂ß₁-deficient mice initiates normally but proceeds at a slower rate, taking about twice as long to occlude the vessel. The contrasting conclusions on the role of ₂ß₁ in experimental arterial thrombosis cannot be reconciled at present with an objective explanation but likely reflect differences in the nature of the vascular lesion, including severity of the endothelial damage and exposure of extracellular matrix components. Thus, similar to ex vivo studies, different thrombogenic surfaces, whether because of the composition or relative proportion of their constituents, seem to elicit distinct pathways of platelet adhesion and activation in vivo.

Glycoprotein VI
This collagen receptor, a member of the immunoglobulin superfamily, is expressed uniquely on megakaryocytes and platelets. It is composed of 2 IgG domains, an O-linked carbohydrate-rich stalk-like region (analogous to the carbohydrate-rich region of GPIb), a transmembrane, and an intracytoplasmic domain.^101,102 Each GPVI molecule is noncovalently coupled to a disulfide-linked FcR- dimer,^103,104 and coassembly of the 2 proteins is required for stable GPVI expression on the membrane (Figure 4).¹⁰⁵ This receptor plays a major role in collagen-induced platelet activation (see the online data supplement). GPVI specifically recognizes the sequence Gly-Pro-Hyp, also known as collagen-related peptide.¹⁰⁶ This peptide assumes a triple-helical conformation in solution and, when cross-linked for stability, is as efficient as collagen in inducing GPVI-mediated platelet activation. A model based on the docking of collagen-related peptide onto the crystal structure of GPVI has been computed to explain in detail the mechanisms of interaction between GPVI and collagen.¹⁰⁷

Unlike ₂ß₁, whose contribution to collagen-dependent platelet responses is still debated, GPVI is generally considered indispensable for normal platelet–collagen interactions.^77,78,108 In ex vivo perfusion experiments, mouse platelets depleted of GPVI by antibody treatment or deficient in GPVI membrane expression because of targeted FcR- gene deletion exhibit a markedly defective adhesion to collagen fibrils.⁷⁷ In similar experiments, mouse platelets congenitally deficient in GPVI exhibit an essentially normal number of initial contacts onto insoluble type I collagen fibers, but activation-dependent spreading associated with irreversible adhesion is abolished, resulting in unstable adhesion and a complete defect of platelet aggregation.⁷⁸ The residual adhesion to collagen exhibited by GPVI-deficient platelets may be explained by a direct function of ₂ß₁ even in the absence of GPVI-dependent activation, or may be initiated by GPIb because plasma VWF binds rapidly to exposed collagen forming a functional unit (Figure 4). In fact, the GPIb-VWF interaction is absolutely required to initiate platelet adhesion to collagen above a threshold shear rate, but can operate at any shear rate.^16,18 In any case, there is no doubt that platelet activation induced by contact with collagen is blocked in the absence of GPVI.

The potential role of GPVI in arterial thrombosis has been evaluated in models of injured carotid artery in the mouse. One group of investigators reported that platelets depleted of GPVI by treatment with an antibody exhibited a reduction in primary adhesion and failed to aggregate on the damaged vessel wall, indicative of an essential role of GPVI in thrombogenesis under arterial flow conditions in vivo.¹⁰⁸ Such results support the concept that GPVI-deficient platelets cannot be normally activated and, thus, cannot adhere irreversibly and form thrombi after the initial tethering mediated by GPIb binding to VWF immobilized onto the exposed collagen.¹⁶ A logical conclusion deriving from these findings is that collagen is an essential if not the only agonist for platelet activation in at least some vascular injury models, a concept reinforced by the demonstration that injected soluble GPVI dimer, presumably by saturating specific binding sites on collagen, protects from the development of acute thrombosis.¹⁰⁹ Other experimental results highlight the role of GPVI in arterial thrombosis and its consequences. Material derived from human athermanous plaques and used as a substrate for ex vivo flow studies has been reported to induce platelet thrombus formation in a GPVI-dependent manner.¹¹⁰ Moreover, it appears that platelets activated by collagen through the GPVI/FcR- pathway play an important role in the process of neointimal hyperplasia after vascular injury.¹¹¹ Finally, fibrillar collagen was found to enhance platelet-mediated thrombin generation through a pathway involving GPVI and supported by ₂ß₁.¹¹² This notwithstanding, other investigators who studied mice deficient in FcR-, thus unable to express GPVI, found that the extent of reduction in thrombogenic response caused by a defective GPVI pathway of interaction with collagen depends on the severity of the vascular injury. In fact, there was no major reduction in thrombus formation in a model of severe arterial injury and a 30% reduction in thrombus growth in a model of mild injury.¹¹³ Of note, inhibition of -thrombin had a more profound antithrombotic effect in mice deficient in GPVI/FcR- than in normal controls, indicating that thrombin can greatly contribute to overcome the functional defect of platelets deficient in collagen-induced activation. In agreement with other experimental findings, these results confirm that the pathways of platelet adhesion/stimulation in injured arteries, likely to influence the severity of thrombotic complications, are determined by the nature of the vascular lesion and of the substrates exposed to blood, including those leading to -thrombin generation.

Additional studies indicate that the contribution of GPVI to platelet thrombus formation depends on the nature of the surface to which blood is exposed and is influenced by still unknown factors. In ex vivo experiments, it has been found that the volume of platelet aggregates is normal when the blood of GPVI^–/– mice is perfused over extracellular matrix deposited by mouse skin fibroblasts.¹⁷ Such a finding, in marked contrast to the results observed on collagen surfaces,⁷⁸ is in agreement with concept that the vessel wall may contain substrates onto which thrombus formation occurs independently of collagen-induced activation, at least as mediated by the known receptors. Moreover, the results of in vivo studies have shown that presently unknown variables influence the role of GPVI in hemostasis and thrombosis. Evaluation of the tail bleeding time⁷⁸ and carotid artery occlusion following a ferric chloride–induced injury¹⁷ has concordantly indicated a dichotomous behavior of GPVI^–/– mice, because a group showed normal results and another a marked prolongation of the bleeding time and absent arterial thrombosis (Figure 5). Of note, the 2 phenotypes were inherited as distinct traits. The likely, but still unproven, explanation for these results is the existence of 1 or more modifier genes that, directly or indirectly, influence the thrombogenic potential of GPVI-deficient platelets. Ongoing studies appear to confirm this hypothesis. Thus, as a consequence of different conditions at a site of vascular injury, the relevance of the role played by GPVI in platelet adhesion and aggregation may vary. It is of interest to note that no exception is known to the required participation of GPIb in the formation of occluding arterial thrombi.¹⁷

View larger version (45K): [in this window] [in a new window]   Figure 5. Ferric chloride–induced carotid artery thrombosis in GPVI-deficient mice. A, Doppler tracing of blood flow in the carotid artery of wild-type and GPVI–/– mice following FeCl3 injury. In the wild-type control shown here, flow begins to decrease 6 minutes after application of FeCl3 to the artery, and a complete occlusion (no flow) is established during the subsequent 3 minutes, with no successive resumption of flow. In the GPVI–/– mouse, in contrast, flow reduction begins between 8 and 9 minutes after FeCl3 application to the artery, and a complete obstruction develops over the next 1 to 2 minutes. Blood flow, however, resumes afterward with alternating cycles of reocclusion (flow decrease) and reopening of the vessel (flow increase), indicating thrombus instability and embolization. Arterial flow is normal at the end of the observation period in the GPVI–/– mouse (no occlusion). B, Bar graph representation of the time to first occlusion, occurrence of embolization, and presence or absence of occlusion at the end of the observation period in 26 GPVI–/– mice. Adapted from Konstantinides et al17 and reprinted with permission.

Platelet Adhesion to Fibronectin
Fibronectin is an essential adhesive substrate in many fundamental biological processes.²⁴ Platelets possess 2 main receptors for this protein, ₅ß₁ and _IIbß₃,¹¹⁴ the latter of which requires activation to function.⁶⁵ It has been known for some time that ₅ß₁ supports platelet adhesion to endothelial extracellular matrix,¹⁶ and more recently direct evidence has been presented that fibronectin may contribute to thrombus formation. A key observation has been that platelets from mice deficient in both VWF and fibrinogen can form thrombi at sites of vascular damage.¹¹⁵ Typically, there is no occlusion at the site of lesion, which can be explained by the required role of VWF in an area of increasing shear rate (see above, under Effects of Hydrodynamic Conditions on Platelet Adhesion and Aggregation), but platelet aggregates are unstable and embolize, causing downstream blockage of flow. In contrast, mice that lack _IIbß₃ are unable to form thrombi under the same conditions, suggesting that a ß₃ ligand different from VWF and fibrinogen can contribute to platelet cohesion. A candidate for this function has been identified by demonstrating that mice with a conditional depletion of plasma fibronectin exhibit a delayed thrombus growth and decreased stability of platelet aggregates,¹¹⁶ suggesting that fibronectin can synergize with VWF and fibrinogen in supporting interplatelet cohesion through activated _IIbß₃. Soluble plasma fibronectin can assemble into fibrillar networks on the surface of fibroblasts, platelets¹¹⁷ and, possibly, other cells, thus providing a substrate that can contribute to the stable attachment of platelet aggregates.²⁵ Fibronectin assembled into fibrillar structures may also support initial platelet adhesion, either directly or indirectly by association with collagen and/or VWF. In experimental conditions ex vivo, purified fibronectin is a rather inefficient substrate for platelet adhesion, and this may reflect the importance of supramolecular assembly with other ligands for the synergistic expression of adhesive function.

Platelet Adhesion to Fibrinogen/Fibrin
In addition to the role in platelet aggregation, which is discussed below, immobilized fibrinogen is a substrate for platelet arrest under flow conditions, selectively mediated by _IIbß₃ in the conformation present on nonactivated platelets.¹⁸ Fibrin, which is a cross-linked insoluble polymer of fibrinogen, retains the ability to support platelet adhesion, and in so doing can synergize with immobilized VWF.¹¹⁸ In experimental perfusion systems, platelets in whole blood adhere in a shear-rate-limited fashion to purified fibrinogen and fibrin, which are progressively less efficient up to a limit of 1000 to 2000 sec^–1.¹⁸ Of note, adherent platelets become activated and spread promptly on surfaces coated with immobilized fibrinogen/fibrin but only small thrombi form in the absence of other adhesive substrates.

Platelet Adhesion to Thrombospondin
Thrombospondins are a family of adhesive proteins,¹¹⁹ of which thrombospondin-1 is contained in platelet -granules. After activation-induced secretion, thrombospondin-1 binds to the platelet membrane and mediates adhesion.¹²⁰ The significance of thrombospondin-1 in platelet thrombus formation is still unclear, although it is intriguing to note that this glycoprotein is abundant in atherosclerotic plaques. In experimental models, immobilized thrombospondin-1 has been shown to support stable platelet attachment up to a shear rate of 4000 sec^–1, a value typically associated with the participation of VWF in the process. Adhesion to thrombospondin-1, however, has been proven to be independent of VWF albeit mediated by GPIb with a minor contribution by GPIV (CD36) only when platelets were activated. The suggestion that thrombospondin-1 may mediate platelet adhesion under arterial flow conditions in lieu of VWF remains to be verified.

Thrombospondin-2 is a relevant constituent of extracellular matrices but is not present in platelets; nonetheless, its absence has been associated with a congenital hemostatic defect in mice.¹²¹ The extent to which this abnormality is directly associated with impaired adhesion is not clear, and alternative explanations are possible as megakaryocytes from mice deficient in thrombospondin-2 produce platelets that are not fully activated by agonist stimulation.¹²² This may exemplify a mechanism for an indirect effect on platelet thrombus formation caused by an adhesive molecule required during megakaryocyte maturation and/or thrombocytopoiesis.

Platelet Adhesion to Laminin
Different forms of laminin are highly expressed in the subendothelial extracellular matrix and, when exposed to blood, are potential substrates for platelet adhesion at sites of vascular injury. The subendothelial forms are laminins 8 (₄ß₁₁) and 10 (₅ß₁₁). Platelets also contain and secrete on activation laminins 8 and 10, as well as laminin 11 (₅ß₂₁).¹²³ It has been reported that platelets can adhere to laminin but are not directly activated following the interaction.¹²³ Other investigators, however, found that human laminin stimulates formation of filopodia and lamellipodia in human and mouse platelets through an adhesion/activation pathway that involves the integrin ₆ß₁, a well-known laminin receptor, and GPVI. Adhesion to laminin appears to depend on ₆ß₁ with no required contribution by GPVI, whereas the latter is necessary for the formation of lamellipodia but not filopodia.¹²⁴ These findings delineate a synergistic role for ₆ß₁ and GPVI as laminin receptors, coupled to distinct activation pathways convergent on the function of the Syk tyrosine kinase.¹²⁴ The relative contribution of adhesion to laminin in platelet thrombus formation remains to be delineated.

	Platelet Adhesion and Thrombus Propagation: Platelet Aggregation

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
Soluble Adhesive Proteins and Their Platelet Receptors in Thrombus Propagation
Following the initial events that lead to adhesion and activation at sites of vascular injury, platelets bind soluble adhesive proteins and form a reactive surface for continuing platelet deposition. Subsequent thrombus growth is therefore strictly dependent on the formation of interplatelet bonds. Aggregation has typically been studied with agonist-stimulated platelets in suspension, without surface interactions, and under stirring conditions that create a turbulent flow with low shear rates.¹²⁵ These studies have led to the assumption that fibrinogen binding to _IIbß₃ is the only interaction relevant for platelet aggregation.¹²⁶ This view has changed with the use of the cone-and-plate viscometer, where platelets in suspension can be exposed to defined levels of shear stress in a laminar flow field. High-shear stress by itself, without addition of exogenous agonists, can lead to platelet aggregation that is mediated by VWF and its 2 membrane receptors.^127,128 Indeed, at elevated shear rates, typically in excess of 5000 sec^–1, VWF binds specifically to platelets in a process that involves sequentially GPIb and _IIbß₃.^128,129 Thus, fibrinogen and VWF, as well as GPIb and _IIbß₃, have distinct but complementary roles in platelet aggregation depending on fluid dynamic conditions.¹²⁹ The study of platelet aggregation in suspension fails to reproduce the correct spatial and temporal sequence of events that occur during thrombus growth in vivo, but there are alternative experimental approaches to measure in real time the 3D growth of platelet aggregates under defined hemodynamic conditions (Figure 1).¹³⁰

Ex vivo perfusion experiments indicate a synergistic role for fibrinogen and VWF in supporting platelet aggregation onto collagen fibrils. Without fibrinogen, thrombi mediated by VWF grow rapidly at high shear rate but are unstable; with both VWF and fibrinogen, thrombi grow more slowly but are stable.¹³⁰ In mice selectively deficient in VWF, platelet adhesion at sites of experimental vascular lesion is delayed, but stable platelet aggregates eventually develop even though arterial occlusion is often impaired.¹¹⁵ In mice selectively deficient in fibrinogen, in contrast, platelet thrombi develop rapidly but detach from the surface and embolize causing vascular occlusion downstream of the lesion.¹¹⁵ Thus, in agreement with the ex vivo perfusion studies outline above, both VWF and fibrinogen are required to ensure stable aggregates. These results provide a plausible explanation for the altered hemostatic properties of platelets from patients with isolated congenital deficiency of either fibrinogen¹³¹ or VWF.¹³² Because neither protein by itself can sustain the development of stable thrombi under pathophysiologically relevant flow conditions, hemostasis cannot be normal unless both are present and functional.

Platelet Adhesion and Thrombus Stabilization
After platelet activation and aggregation have occurred in response to a vascular lesion, distinct adhesive mechanisms become operative that consolidate the stability of the forming thrombus. The identification and characterization of the molecules involved in these processes have become the focus of increasing attention in recent years because of their seemingly important influence on hemostatic efficiency and, in pathological conditions, arterial thrombotic complications (see the online data supplement).

	Conclusions

Top Abstract Introduction Platelet Adhesion and Platelet... Effects of Hydrodynamic... Initial Platelet Adhesion to... Platelet Adhesion and Thrombus... Conclusions References

 
The last few years have witnessed major advances in our understanding of the mechanisms that support platelet thrombus formation in flowing blood. The results of ex vivo flow experiments and intravital microscopy studies have shed new light on the processes underlying hemostasis and thrombosis. Animal models with targeted gene deletions or mutations have greatly contributed to these advances and will certainly provide more insights in the future. Significant progress in genomics and proteomics has generated information relevant to elucidating the integrated processes that link platelet–substrate interactions and signaling pathways to thrombus growth and stability. Finally, the advent of improved drug development technologies is likely to permit the translation of this fundamental knowledge into more efficient therapeutic approaches to prevent excessive bleeding and thrombosis.

	Acknowledgments

 
We gratefully acknowledge the many contributions that all of the colleagues in the laboratory of Z.M.R. have made to the original work discussed here.

Sources of Funding

The original work by the authors described in this review was supported by grants from the National Heart Lung and Blood Institute of the NIH (to Z.M.R.).

Disclosures

None.

	Footnotes

 
Original received January 2, 2007; revision received March 13, 2007; accepted April 11, 2007.

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