Phosphoproteome of resting human platelets

Beside their main physiological function in hemostasis, platelets are also highly involved in pathological processes, such as atherothrombosis and inflammation. During hemostasis, binding of adhesive substrates to tyrosine-kinase-linked adhesion receptors and/or soluble agonists to G-protein coupled receptors leads to a cascade of intracellular signaling processes based on substrate (de)phosphorylation. The same mechanisms are involved in platelet activation at sites of atherosclerotic plaque rupture, contributing to vessel occlusion and consequently to pathologic states, such as myocardial infarction, stroke, or peripheral artery disease. To gain a deeper insight into platelet function, we analyzed the phosphoproteome of resting platelets and identified 564 phosphorylation sites from more than 270 proteins, of which many have not been described in platelets before. Among those were several unknown potential protein kinase A (PKA) and protein kinase G (PKG) substrates. Because platelet inhibition is tightly regulated especially by PKA and PKG activity, these proteins may represent important new targets for cardiovascular research. Thus, our finding that GPIbalpha is phosphorylated at Ser603 in resting platelets may represent a novel mechanism for the regulation of one of the most important platelet receptor (GPIb-IX-V) mediated signaling pathways by PKA/PKG.     

A novel role for phospholipase D as an endogenous negative regulator of platelet sensitivity

Platelet aggregation, secretion and thrombus formation play a critical role in primary hemostasis to prevent excessive blood loss. On the other hand, uncontrolled platelet activation leads to pathological thrombus formation resulting in myocardial infarction or stroke. Stimulation of heterotrimeric G-proteins by soluble agonists or immunoreceptor tyrosine based activation motif-coupled receptors that interact with immobilized ligands such as the collagen receptor glycoprotein (GP) VI lead to the activation of phospholipases that cleave membrane phospholipids to generate soluble second messengers. Platelets contain the phospholipases (PL) D1 and D2 which catalyze the hydrolysis of phosphatidylcholine to generate the second messenger phosphatidic acid (PA). The production of PA is abrogated by primary alcohols that have been widely used for the analysis of PLD-mediated processes. However, it is not clear if primary alcohols effectively reduce PA generation or if they induce PLD-independent cellular effects. In the present study we made use of the specific PLD inhibitor 5-fluoro-2-indolyl des-chlorohalopemide (FIPI) and show for the first time, that FIPI enhances platelet dense granule secretion and aggregation of human platelets. Further, FIPI has no effect on cytosolic Ca(2+) activity but needs proper Rho kinase signaling to mediate FIPI-induced effects on platelet activation. Upon FIPI treatment the phosphorylation of the PKC substrate pleckstrin was prominently enhanced suggesting that FIPI affects PKC-mediated secretion and aggregation in platelets. Similar effects of FIPI were observed in platelets from mouse wild-type and Pld1(-/-) mice pointing to a new role for PLD2 as a negative regulator of platelet sensitivity.     

Platelet Mechanosensing of Collagen Matrices

During vascular injury, platelets adhere to exposed subendothelial proteins, such as collagen, on the blood vessel walls to trigger clot formation. Although the biochemical signalings of platelet-collagen interactions have been well characterized, little is known about the role microenvironmental biomechanical properties, such as vascular wall stiffness, may have on clot formation. To that end, we investigated how substrates of varying stiffness conjugated with the same concentration of Type I collagen affect platelet adhesion, spreading, and activation. Using collagen-conjugated polyacrylamide (PA) gels of different stiffnesses, we observed that platelets do in fact mechanotransduce the stiffness cues of collagen substrates, manifesting in increased platelet spreading on stiffer substrates. In addition, increasing substrate stiffness also increases phosphatidylserine exposure, a key aspect of platelet activation that initiates coagulation on the platelet surface. Mechanistically, these collagen substrate stiffness effects are mediated by extracellular calcium levels and actomyosin pathways driven by myosin light chain kinase but not Rho-associated protein kinase. Overall, our results improve our understanding of how the mechanics of different tissues and stroma affect clot formation, what role the increased vessel wall stiffness in atherosclerosis may directly have on thrombosis leading to heart attacks and strokes, and how age-related increased vessel wall stiffness affects hemostasis and thrombosis.     

G13 is an essential mediator of platelet activation in hemostasis and thrombosis

Platelet activation at sites of vascular injury is essential for primary hemostasis, but also underlies arterial thrombosis leading to myocardial infarction or stroke. Platelet activators such as adenosine diphosphate, thrombin or thromboxane A(2) (TXA(2)) activate receptors that are coupled to heterotrimeric G proteins. Activation of platelets through these receptors involves signaling through G(q), G(i) and G(z) (refs. 4-6). However, the role and relative importance of G(12) and G(13), which are activated by various platelet stimuli, are unclear. Here we show that lack of Galpha(13), but not Galpha(12), severely reduced the potency of thrombin, TXA(2) and collagen to induce platelet shape changes and aggregation in vitro. These defects were accompanied by reduced activation of RhoA and inability to form stable platelet thrombi under high shear stress ex vivo. Galpha(13) deficiency in platelets resulted in a severe defect in primary hemostasis and complete protection against arterial thrombosis in vivo. We conclude that G(13)-mediated signaling processes are required for normal hemostasis and thrombosis and may serve as a new target for antiplatelet drugs.     

Pre-activated blood platelets and a pro-thrombotic phenotype in APP23 mice modeling Alzheimer's disease

Platelet activation and thrombus formation play a critical role in primary hemostasis but also represent a pathophysiological mechanism leading to acute thrombotic vascular occlusions. Besides, platelets modulate cellular processes including inflammation, angiogenesis and neurodegeneration. On the other hand, platelet activation and thrombus formation are altered in different diseases leading to either bleeding complications or pathological thrombus formation. For many years platelets have been considered to play a role in neuroinflammatory diseases such as Alzheimer's disease (AD). AD is characterized by deposits of amyloid-β (Aβ) and strongly related to vascular diseases with platelets playing a critical role in the progression of AD because exposure of platelets to Aβ induces platelet activation, platelet Aβ release, and enhanced platelet adhesion to collagen in vitro and at the injured carotid artery in vivo. However, the molecular mechanisms and the relation between vascular pathology and amyloid-β plaque formation in the pathogenesis of AD are not fully understood. Compelling evidence is suggestive for altered platelet activity in AD patients. Thus we analyzed platelet activation and thrombus formation in aged AD transgenic mice (APP23) known to develop amyloid-β deposits in the brain parenchyma and cerebral vessels. As a result, platelets are in a pre-activated state in blood of APP23 mice and showed strongly enhanced integrin activation, degranulation and spreading kinetics on fibrinogen surfaces upon stimulation. This enhanced platelet signaling translated into almost unlimited thrombus formation on collagen under flow conditions in vitro and accelerated vessel occlusion in vivo suggesting that these mice are at high risk of arterial thrombosis leading to cerebrovascular and unexpectedly to cardiovascular complications that might be also relevant in AD patients.     

The role of heterotrimeric G proteins in platelet activation

Activation of platelets plays a central role in hemostasis as well as in various thromboembolic diseases like myocardial infarction or stroke. Most platelet activating stimuli function through receptors which couple to heterotrimeric G proteins of the Gi, Gq and G12 families. Recent studies have elucidated the roles of individual G proteins in the regulation of platelet functions like shape change, aggregation and granule secretion. The signaling pathways mediated by heterotrimeric G proteins operate synergistically to induce a full activation of platelets. This review summarizes recent progress in the understanding of upstream regulation of platelet activation through G protein-coupled receptors.     

Multi-omics approaches to study platelet mechanisms

Platelets are small anucleate cell fragments (2–4 μm in diameter) in the blood, which play an essential role in thrombosis and hemostasis. Genetic or acquired platelet dysfunctions are linked to bleeding, increased risk of thromboembolic events and cardiovascular diseases. Advanced proteomic approaches may pave the way to a better understanding of the roles of platelets in hemostasis, and pathophysiological processes such as inflammation, metastatic spread and thrombosis. Further insights into the molecular biology of platelets are crucial to aid drug development and identify diagnostic markers of platelet activation. Platelet activation is known to be an extremely rapid process and involves multiple post-translational mechanisms at sub second time scale, including proteolysis and phosphorylation. Multi-omics technologies and biochemical approaches can be exploited to precisely probe and define these posttranslational pathways. Notably, the absence of a nucleus in platelets significantly reduces the number of present proteins, simplifying mass spectrometry-based proteomics and metabolomics approaches.     

Adaptor proteins of blood platelets

Adaptor proteins play a pivotal role in the regulation of signal transduction events elicited after the engagement of cell surface receptors. Platelets exhibit a number of integral membrane receptors capable of initiating a cellular response. These include collagen receptors, von Willebrand factor receptors, the fibrinogen receptor, and a number of G-protein coupled receptors, such as those for thrombin and ADP. The primary function of platelet receptors is the translation of externally applied signals into appropriate responses leading to platelet activation being a prerequisite for normal hemostasis. Multitude of signalling pathways described in platelets is based on the interaction of compounds of many different categories, such as transmembrane receptors, protein kinases, protein phoshatases, G-proteins, transmembrane and cytosolic adaptor proteins, phosphoinositides, cyclic AMP or GMP. Adaptor proteins lack intrinsic effector function, but contain distinct molecular domains, which mediate protein-protein and protein-lipid interactions. These molecules thus serve as a scaffolding, around which effectors and their substrates are assembled into three-dimensional signaling complexes. Adaptor proteins integrate receptor-mediated signals at intracellular levels and couple signaling receptors to cytosolic signaling pathways. While the function of adaptor proteins is well established in immune cells, the knowledge about their role in platelet activation is still at the onset Over the last decade numerous adaptor proteins have been identified in platelets and shown to be involved in accurate assembly of intracellular signaling complexes. Collagen-induced platelet intracellular signaling through GPVI resembles the functional response of B- and T-cell antigen receptors and is the best described in the literature. This review focuses on the structure and functional role of the most extensively studied adaptor proteins during platelet activation induced by physiological agonists.     

Collagen Can Selectively Trigger a Platelet Secretory Phenotype via Glycoprotein VI

Platelets are not only central actors of hemostasis and thrombosis but also of other processes including inflammation, angiogenesis, and tissue regeneration. Accumulating evidence indicates that these “non classical” functions of platelets do not necessarily rely on their well-known ability to form thrombi upon activation. This suggests the existence of non-thrombotic alternative states of platelets activation. We investigated this possibility through dose-response analysis of thrombin- and collagen-induced changes in platelet phenotype, with regards to morphological and functional markers of platelet activation including shape change, aggregation, P-selectin and phosphatidylserine surface expression, integrin activation, and release of soluble factors. We show that collagen at low dose (0.25 µg/mL) selectively triggers a platelet secretory phenotype characterized by the release of dense- and alpha granule-derived soluble factors without causing any of the other major platelet changes that usually accompany thrombus formation. Using a blocking antibody to glycoprotein VI (GPVI), we further show that this response is mediated by GPVI. Taken together, our results show that platelet activation goes beyond the mechanisms leading to platelet aggregation and also includes alternative platelet phenotypes that might contribute to their thrombus-independent functions.     

Antioxidants change platelet responses to various stimulating events

The role of platelets in hemostasis may be influenced by alteration of the platelet redox state—the presence of antioxidants and the formation of reactive oxygen and nitrogen species. We investigated the effects of two antioxidants, resveratrol and trolox, on platelet activation. Trolox and resveratrol inhibited aggregation of washed platelets and platelet-rich plasma activated by ADP, collagen, and thrombin receptor-activating peptide. Resveratrol was a more effective agent in reducing platelet static and dynamic adhesion in comparison with trolox. The antioxidant capacity of resveratrol was, however, the same as that of trolox. After incubation of platelets with antioxidants, the resveratrol intraplatelet concentration was about five times lower than the intracellular concentration of trolox. Although both antioxidants comparably lowered hydroxyl radical and malondialdehyde production in platelets stimulated with collagen, TxB₂ levels were decreased by resveratrol much more effectively than by trolox. Cyclooxygenase 1 was inhibited by resveratrol and not by trolox. Our data indicate that antioxidants, apart from nonspecific redox or radical-quenching mechanisms, inhibit platelet activation also by specific interaction with target proteins. The results also show the importance of studying platelet activation under conditions of real blood flow in contact with reactive surfaces, e.g., using dynamic adhesion experiments.     

The Blood Platelet as a Model for Regulating Blood Coagulation on Cell Surfaces and Its Consequences

Platelets actively participate in regulating thrombin production following physical or chemical injury to blood vessels. Injury to blood vessels initiates activation of the large numbers of platelets that appear in the subendothelium where they become exposed to tissue factor and to molecules adhesive for platelets and normally found in the extracellular matrix. The complex of plasma factor VIIa with extravascular tissue factor both initiates and localizes thrombin production on platelets and on extravascular cells. Thrombin production at these sites in turn enhances platelet activation and the subsequent hemostatic plug formation to minimize bleeding. Thrombin production and platelet activation also initiate the process of wound healing requiring thrombin-dependent cell activation and platelet-dependent formation of new blood vessels (angiogenesis). Activated platelets release from their storage granules several proteins and other factors that regulate local thrombin formation and the responses of blood vessel cells to injury to assure hemostasis and effective wound healing. Failure to localize and adequately regulate thrombin production and/or platelet activation can have pathological consequences, including the development and propagation of atherosclerosis and enhancement of tumor development. The primary basis for the pathological consequences of the failure to adequately regulate thrombin production is that the multi-functional thrombin activates several types of cells to initiate their mitogenesis. Mitogenesis precedes many of the undesirable consequences of poorly regulated thrombin production and platelet activation. In addition, activated platelets release a variety of products which influence the functions of several cell types to the extent that inadequate regulation of platelet activation (by excessive thrombin production) could contribute to the pathogenesis of acute and chronic arterial thrombosis and to tumor development. Activated platelets participate in tumor development by releasing several factors that positively (and negatively) regulate blood vessel formation.     

Rhinocetin, a venom-derived integrin-specific antagonist inhibits collagen-induced platelet and endothelial cell functions

Snaclecs are small non-enzymatic proteins present in viper venoms reported to modulate hemostasis of victims through effects on platelets, vascular endothelial, and smooth muscle cells. In this study, we have isolated and functionally characterized a snaclec that we named "rhinocetin" from the venom of West African gaboon viper, Bitis gabonica rhinoceros. Rhinocetin was shown to comprise α and β chains with the molecular masses of 13.5 and 13 kDa, respectively. Sequence and immunoblot analysis of rhinocetin confirmed this to be a novel snaclec. Rhinocetin inhibited collagen-stimulated activation of human platelets in a dose-dependent manner but displayed no inhibitory effects on glycoprotein VI (collagen receptor) selective agonist, CRP-XL-, ADP-, or thrombin-induced platelet activation. Rhinocetin antagonized the binding of monoclonal antibodies against the α2 subunit of integrin α2β1 to platelets and coimmunoprecipitation analysis confirmed integrin α2β1 as a target for this venom protein. Rhinocetin inhibited a range of collagen-induced platelet functions such as fibrinogen binding, calcium mobilization, granule secretion, aggregation, and thrombus formation. It also inhibited integrin α2β1-dependent functions of human endothelial cells. Together, our data suggest rhinocetin to be a modulator of integrin α2β1 function and thus may provide valuable insights into the role of this integrin in physiological and pathophysiological scenarios, including hemostasis, thrombosis, and envenomation.     

Platelet adhesion receptors and (patho)physiological thrombus formation

In thrombus formation associated with hemostasis or thrombotic disease, blood platelets first undergo a rapid transition from a circulating state to an adherent state, followed by activation and aggregation. Under flow conditions in the bloodstream, this process potentially involves platelet-platelet, platelet-endothelium, platelet-subendothelial matrix, and platelet-leukocyte interactions. Specific adhesion receptors on platelets mediate these interactions, by engaging counter-receptors on other cells, or noncellular ligands in the plasma or matrix. The glycoprotein (GP) Ib-IX-V complex on platelets initiates adhesion at high shear stress by binding the adhesive ligand, von Willebrand Factor (vWF). GP Ib-IX-V may also mediate platelet-endothelium or platelet-leukocyte adhesion, by recognition of P-selectin or Mac-1, respectively. Other membrane glycoproteins, such as the collagen receptor GP VI, may trigger platelet activation at low shear rates. Engagement of GP Ib-IX-V or GP VI leads ultimately to platelet aggregation mediated by the integrin, alphaIIbbeta3 (GP IIb-IIIa). This review will focus on recent advances in understanding structure-activity relationships of GP Ib-IX-V, its role in initiating thrombus formation, and its emerging relationships with other vascular cell adhesion receptors.     

Inhibition of thrombin-induced platelet activation by leupeptin. Implications for the participation of calpain in the initiation of platelet activation

Inhibitors of calcium-dependent proteases (calpains) such as leupeptin and antipain have been shown to selectively inhibit platelet activation by thrombin. Based upon this observation, it has been proposed that calpains play a role in the initiation of platelet activation. In the present studies, we have examined the effect of leupeptin on the earliest known event in thrombin-induced platelet activation: the interaction between the agonist, its receptors, and the guanine nucleotide-binding proteins which stimulate phospholipase C (Gp) and inhibit adenylyl cyclase (Gi). We found that leupeptin inhibited thrombin's ability to stimulate phosphoinositide hydrolysis, suppress cAMP formation, and dissociate Gp and Gi into subunits. Leupeptin had no effect, however, on the same responses to other agonists or on thrombin binding to platelets. Although these observations might suggest, as others have concluded, that calpain is involved in the initiation of platelet activation by thrombin, we also found that: 1) substituting platelet membranes for intact platelets and decreasing the free Ca2+ concentration below the threshold required for calpain activation did not diminish the effects of leupeptin on phosphoinositide hydrolysis and cAMP formation, 2) washing the platelets after incubation with leupeptin reversed the effects of the inhibitor, 3) permeabilizing the platelets with saponin did not enhance the inhibitory effects of leupeptin, and 4) leupeptin inhibited the proteolysis of fibrinogen and the hydrolysis of S2238 by thrombin. Similar results in these assays were obtained with antipain. Therefore, our observations suggest that the inhibition of platelet activation by leupeptin is due to a direct interaction with thrombin and need not reflect a role for calpain in the initiation of platelet activation.     

Integrin alpha 2-deficient mice develop normally, are fertile, but display partially defective platelet interaction with collagen.

The integrin alpha(2)-subunit was ablated in mice by targeted deletion of the ITGA2 gene. alpha(2)-Deficient animals develop normally, are fertile, and reproduce. Surprisingly, no obvious anatomical or histological differences were observed in mutant mice. Besides its significance in tissue morphogenesis, integrin alpha(2)beta(1) has been reported to play a major role in hemostasis by mediating platelet adhesion and activation on subendothelial collagen. To define its role in hemostasis, alpha(2)-deficient platelets were analyzed for their capacity to adhere to and aggregate in response to fibrillar or soluble collagen type I. We show that aggregation of alpha(2)-deficient platelets to fibrillar collagen is delayed but not reduced, whereas aggregation to enzymatically digested soluble collagen is abolished. Furthermore, alpha(2)-deficient platelets normally adhere to fibrillar collagen. However, in the presence of an antibody against GPVI (activating platelet collagen receptor), adhesion of alpha(2)-deficient but not wild type platelets is abrogated. These results demonstrate that integrin alpha(2)beta(1) significantly contributes to platelet adhesion to (fibrillar) collagen, which is further confirmed by the abolished adhesion of alpha(2)-deficient platelets to soluble collagen. Thus, alpha(2)beta(1) plays a supportive rather than an essential role in platelet-collagen interactions. These results are in agreement with the observation that alpha(2)beta(1)-deficient animals suffer no bleeding anomalies.     

Extracellular Matrix Proteins in Hemostasis and Thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell–matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.The balance between hemostasis and thrombosis relies on a finely tuned adhesive response of blood platelets. Inadequate adhesion leads to bleeding, whereas excessive or inappropriate adhesion leads to thrombosis. Resting platelets are nonadhesive anuclear discs and do not interact with the vessel wall, but they have a plethora of receptors that sense activating signals (agonists) of various sorts. The activating signals include soluble factors such as thrombin, adenosine diphosphate (ADP), and epinephrine, all of which act on G-protein-coupled receptors (GPCRs) on the platelets. In addition, certain receptors for extracellular matrix (ECM) proteins (e.g., GPIb, GPVI, and some integrins) can also act as activating receptors. These diverse receptors trigger intracellular signaling pathways that activate (1) actin assembly leading to cell shape change and extension of filopodia; (2) exocytosis of secretory granules that release additional platelet agonists as well as adhesive ECM proteins; and (3) activation of additional cell-surface receptors such as the major platelet-specific integrin, αIIbβ3, that contribute further to the adhesion and aggregation of activated platelets. Thus, the interactions of platelet-ECM adhesion receptors with ECM proteins from the vessel wall, from the plasma, and from the platelets themselves, are central to both the initial adhesion and the subsequent activation and aggregation of platelets (Varga-Szabo et al. 2008). These adhesive interactions, together with coagulation (to which platelets also contribute), generate the fibrin clot, essentially a facultative ECM that forms the initial occlusion of the damaged vessel but also serves as a subsequent ECM substrate for wound healing. In this article, we will review what is known about the roles of ECM proteins and their receptors in platelet adhesion and aggregation, summarize the roles of the clot and provisional ECM in subsequent wound healing, point out various unanswered questions, and discuss briefly the contributions of the relevant cell–ECM interactions to disease and the potential for therapeutic interventions.     

RGS10 Negatively Regulates Platelet Activation and Thrombogenesis

Regulators of G protein signaling (RGS) proteins act as GTPase activating proteins to negatively regulate G protein-coupled receptor (GPCR) signaling. Although several RGS proteins including RGS2, RGS16, RGS10, and RGS18 are expressed in human and mouse platelets, the respective unique function(s) of each have not been fully delineated. RGS10 is a member of the D/R12 subfamily of RGS proteins and is expressed in microglia, macrophages, megakaryocytes, and platelets. We used a genetic approach to examine the role(s) of RGS10 in platelet activation in vitro and hemostasis and thrombosis in vivo. GPCR-induced aggregation, secretion, and integrin activation was much more pronounced in platelets from Rgs10^-/- mice relative to wild type (WT). Accordingly, these mice had markedly reduced bleeding times and were more susceptible to vascular injury-associated thrombus formation than control mice. These findings suggest a unique, non-redundant role of RGS10 in modulating the hemostatic and thrombotic functions of platelets in mice. RGS10 thus represents a potential therapeutic target to control platelet activity and/or hypercoagulable states.     

Disruption of the mouse mu-calpain gene reveals an essential role in platelet function

Conventional calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, cell proliferation, apoptosis, cell motility, and hemostasis. There are two forms of conventional calpains: the mu-calpain, or calpain I, which requires micromolar calcium for half-maximal activation, and the m-calpain, or calpain II, which functions at millimolar calcium concentrations. We evaluated the functional role of the 80-kDa catalytic subunit of mu-calpain by genetic inactivation using homologous recombination in embryonic stem cells. The mu-calpain-deficient mice are viable and fertile. The complete deficiency of mu-calpain causes significant reduction in platelet aggregation and clot retraction but surprisingly the mutant mice display normal bleeding times. No detectable differences were observed in the cleavage pattern and kinetics of calpain substrates such as the beta3 subunit of alphaIIbbeta3 integrin, talin, and ABP-280 (filamin). However, mu-calpain null platelets exhibit impaired tyrosine phosphorylation of several proteins including the beta3 subunit of alphaIIbbeta3 integrin, correlating with the agonist-induced reduction in platelet aggregation. These results provide the first direct evidence that mu-calpain is essential for normal platelet function, not by affecting the cleavage of cytoskeletal proteins but by potentially regulating the state of tyrosine phosphorylation of the platelet proteins.     

Monitoring modulators of platelet aggregation in a microtiter plate assay

Platelets play a central role in maintaining biological hemostasis. Inappropriate platelet activation is responsible for thrombotic diseases such as myocardial infarction and stroke. Therefore, novel agents that can inhibit platelet activation are necessary. However, assays that monitor platelet aggregation are generally time-consuming and require high volumes of blood and specialized equipment. Therefore, a medium- to high-throughput assay that can monitor platelet aggregation would be considered useful. Such an assay should be sensitive, comparable to the "gold standard" assay of platelet aggregometry, and able to monitor multiple samples simultaneously but with low assay volumes. We have developed such a microtiter assay. It can assay an average of 60 independent treatments per 60 ml blood donation and demonstrates greater sensitivity than the current gold standard assay, namely platelet aggregation in stirring conditions in a platelet aggregometer. The microtiter plate (MTP) assay can detect known inhibitors of platelet function such as indomethacin, aspirin, and ReoPro. It is highly reproducible when using standard doses of agonists such as thrombin receptor-activating peptide (20 microM) and collagen (0.19 mg/ml). Finally, the MTP assay is rapid and sensitive and can detect unknown platelet-modulating agents from a library of compounds.