Pathologic high shear stress induces apoptosis events in human platelets

Mitochondrially mediated integrin αIIbβ3 protein inactivation limits thrombus growth

When platelets are strongly stimulated, a procoagulant platelet subpopulation is formed that is characterized by phosphatidylserine (PS) exposure and epitope modulation of integrin αIIbβ3 or a loss of binding of activation-dependent antibodies. Mitochondrial permeability transition pore (mPTP) formation, which is essential for the formation of procoagulant platelets, is impaired in the absence of cyclophilin D (CypD). Here we investigate the mechanisms responsible for these procoagulant platelet-associated changes in integrin αIIbβ3 and the physiologic role of procoagulant platelet formation in the regulation of platelet aggregation. Among strongly stimulated adherent platelets, integrin αIIbβ3 epitope changes, mPTP formation, PS exposure, and platelet rounding were closely associated. Furthermore, platelet mPTP formation resulted in a decreased ability to recruit additional platelets. In the absence of CypD, integrin αIIbβ3 function was accentuated in both static and flow conditions, and, in vivo, a prothrombotic phenotype occurred in mice with a platelet-specific deficiency of CypD. CypD-dependent proteolytic events, including cleavage of the integrin β3 cytoplasmic domain, coincided closely with integrin αIIbβ3 inactivation. Calpain inhibition blocked integrin β3 cleavage and inactivation but not mPTP formation or PS exposure, indicating that integrin inactivation and PS exposure are mediated by distinct pathways subsequent to mPTP formation. mPTP-dependent alkalinization occurred in procoagulant platelets, suggesting a possible alternative mechanism for enhancement of calpain activity in procoagulant platelets. Together, these results indicate that, in strongly stimulated platelets, mPTP formation initiates the calpain-dependent cleavage of integrin β3 and associated regulatory proteins, resulting in integrin αIIbβ3 inactivation, and demonstrate a novel CypD-dependent negative feedback mechanism that limits platelet aggregation and thrombotic occlusion.

Oxidized low-density lipoprotein-induced CD147 expression and its inhibition by high-density lipoprotein on platelets in vitro

INTRODUCTION

Matrix metalloproteinases (MMPs) are believed to progressively degrade the collagenous components of the protective fibrous cap, leading to atherosclerotic plaque rupture or destabilization. Oxidized low-density lipoprotein (ox-LDL) enhances the release of CD147, known as the extracellular MMP inducer, from coronary smooth muscle cells. However, whether ox-LDL can induce platelet CD147 expression is unknown. Therefore, we investigated the influence of ox-LDL and high-density lipoprotein (HDL) on CD147 expression on human platelets.

MATERIALS AND METHODS

Washed platelets were incubated with ox-LDL (or native LDL) and HDL or anti-LOX-1 monoclonal antibody prior to incubation with ox-LDL. In parallel, buffer (PBS) was added to washed platelets as a control. The expression levels of CD147, CD62P, CD63 and Annexin V were assessed by flow cytometry, and soluble CD147 from the platelets was assessed by an enzyme-linked immunosorbent assay. Laser scanning microscopy (LSM) and transmission electron microscopy (TEM) were used to visualize the morphological changes and granule release, respectively, from the platelets.

RESULTS

Platelets treated with ox-LDL exhibited a significant increase in the expression of CD147 (or Annexin V), followed by increases in CD62P and CD63, compared with the control group. In contrast, HDL or anti-LOX-1 monoclonal antibody decreased these effects. The expression of soluble CD147 increased as the concentration of ox-LDL used to treat the platelets increased. After exposure to ox-LDL, morphological changes and granule release in the platelets were visualized by LSM and TEM. Additionally, the TEM revealed that HDL inhibits alpha-granule release.

CONCLUSIONS

In platelets, ox-LDL stimulates the release of CD147 via binding to LOX-1, whereas HDL inhibits this effect. This finding could provide new insights concerning the influence of ox-LDL and HDL on plaque stability by the up-regulation of CD147 on platelets.

Concurrent and separate inside-out transition of platelet apoptosis and activation markers to the platelet surface

The cell plasma membrane is tightly coupled with the vital processes of apoptosis and activation. In the current study, we investigated exposure of the apoptosis marker phosphatidylserine (PS) and activation marker P-selectin (CD62) on the plasma membrane of anucleate platelets. We found that, depending on triggering stimuli, the plasma membrane of human platelets may exist in four states with predominant exposure of (i) PS but not CD62 (75·9 ± 2·8% of total cells), (ii) CD62 but not PS (86·2 ± 1·3%), (iii) both PS and CD62 (89·6 ± 1·0%) or (iv) neither PS nor CD62 (87·9-97·5%), when platelets were treated at optimal conditions with pro-apoptotic BH3 mimetic ABT-737, thrombin, calcium ionophore A23187 or control diluents, respectively. The dynamics of PS exposure induced by ABT-737 is a slow temperature-dependent process requiring 90 min treatment at 37°C rather than at room temperature for obtaining high levels of PS exposure. In contrast, thrombin-induced CD62 exposure and A23187-induced PS and CD62 exposure showed fast temperature-independent dynamics. This model of selective and concurrent stimulation of PS and/or CD62 transition to the platelet surface provides an experimental horizon for elucidating the roles of plasma membrane markers of platelet apoptosis and activation in platelet clearance.

Platelet mitochondrial function: from regulation of thrombosis to biomarker of disease

Circulating blood platelets contain small numbers of fully functional mitochondria. Accumulating evidence demonstrates that these mitochondria regulate the pro-thrombotic function of platelets through not only energy generation, but also redox signalling and the initiation of apoptosis. Beyond its regulation of haemostasis, platelet mitochondrial function has also traditionally been used to identify and study mitochondrial dysfunction in human disease, owing to the easy accessibility of platelets compared with other metabolically active tissues. In the present article, we provide a brief overview of what is currently known about the function of mitochondria in platelets and review how platelet mitochondria have been used to study mitochondrial function in human disease.

Activation induced morphological changes and integrin αIIbβ3 activity of living platelets

Platelets are essential in hemostasis. Upon activation they undergo a shape-change accompanied with receptor presentation. Atomic force microscopy (AFM) imaging and single molecule force spectroscopy (SMFS) were used as powerful tools for exploring morphological changes as well as receptor activities of platelets. Imaging time series was accomplished with and without fixation steps at the single platelet level. Hereby the response of mechanical stimulation of the platelet by the AFM cantilever tip was directly observed. We demonstrate that living and fixed platelets develop filopodia after a short activation time followed by their disappearance including cellular bleb formation. Thereafter a second filopodia formation (filopodia extrusion) was observed; those filopodia subsequently disappeared again, and finally platelets detached from the support due to cell death. We determined the influence of mechanical stress on the chronology of morphological changes of platelets and demonstrated shear force induced filopodia formation. Through recordings over several hours, topographical AFM images over the full platelet lifetime - from early activation up to apoptosis - are presented. SMFS measurements on living platelets allowed determining the activation state of the most prominent membrane receptor integrin αIIbβ3 at all different phases of activation. αIIbβ3 was fully activated, independent of the morphological state.

Selective triggering of platelet apoptosis, platelet activation or both

Anucleate platelets perform two fundamental processes, activation and apoptosis. We elaborated an approach for selective and concurrent stimulation of platelet apoptosis and/or activation, processes important in haemostasis and platelet clearance. Human platelets were treated with BH3 mimetic ABT-737, thrombin, calcium ionophore A23187 and matched diluents. Apoptosis was determined as mitochondrial inner membrane potential (ΔΨm) depolarization and activation as P-selectin exposure. At optimal treatment conditions (90-180 min, 37°C), ABT-737 predominantly induced apoptosis, when 77-81% platelets undergo only ΔΨm depolarization. The ABT-737 impact on ΔΨm depolarization is strongly time- and temperature-dependent, and much higher at 37°C than at room temperature. In contrast, when platelets were treated with thrombin for 15-90 min at either temperature, activation-only was predominantly (79-85%) induced, whereas A23187 triggers both apoptosis and activation (73-81%) when platelets were treated for 15-60 min at 37°C or 15-90 min at room temperature. These data demonstrate that, depending on the triggering stimulus, platelets predominantly undergo ΔΨm depolarization-only, P-selectin exposure-only, or both responses, indicating that platelet apoptosis and activation are different phenomena driven by different mechanisms. The described model provides a basis for studying differential pharmacological manipulation of platelet apoptosis and activation and their role in haemostasis, thrombosis and platelet clearance.

Role of platelets in neurodegenerative diseases: a universal pathophysiology

Platelets play an important role in a variety of disorders, namely, cardiovascular, psychosomatic, psychiatric, thrombosis, HIV/AIDS in addition to various neurodegenerative diseases (NDDs). Recent evidence indicates that platelet react to diverse stressors, thereby offering an interesting vantage point for understanding their potential role in contemporary medical research. This review addresses the possible role of platelets as a systemic probe in various NDDs, such as amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, etc. The current review based on published literature, describes a probable link between platelets and pathophysiology of various NDDs. It also discusses how platelets epitomize ultrastructural, morphological, biochemical and molecular changes, highlighting their emerging role as systemic tools in different NDDs.

Aspirin Induces platelet apoptosis

Aspirin is widely used in the treatment of a number of clinical conditions. Although aspirin is being thought to be a relatively "safe" medicine, it also has some side effects, particularly the risk of bleeding which may be severe and lead to death. The mechanisms, however, are not totally understood. It has been reported recently that aspirin induces apoptosis in many cell types. Thus, the aim of the current study is to explore whether aspirin induces platelet apoptosis. The data show that mitochondrial transmembrane potential (ΔΨm) depolarizations and phosphatidylserine (PS) exposures were dose-dependently induced by aspirin in platelets. To further confirm that aspirin incurs platelet apoptosis, caspase-3 activity was measured in platelets, and the result indicated that aspirin induced caspase-3 activation. Furthermore, the mean volume of platelets incubated with aspirin was obviously reduced. Caspase inhibitor z-VAD-fmk inhibited aspirin induced apoptotic platelet shrinkage and ΔΨm depolarization, but had no effect on PS exposure. In addition, platelets incubated with cyclooxygenase inhibitor indomethacin did not incur ΔΨm depolarazation and PS exposure. Taken together, the data indicate that aspirin induces platelet apoptosis via caspase-3 activation.

Mechanisms of andrographolide-induced platelet apoptosis in human platelets: regulatory roles of the extrinsic apoptotic pathway

Andrographolide, a novel nuclear factor-κB (NF-κB) inhibitor, is isolated from the leaves of Andrographis paniculata. Platelet activation is relevant to a variety of coronary heart diseases. Our recent studies revealed that andrographolide possesses potent antiplatelet activity by inhibition of the p38 MAPK/(●) HO-NF-κB-ERK2 cascade. Although platelets are anucleated cells, apoptotic machinery apparatus recently has been found to regulate platelet activation and limit platelet lifespan. Therefore, we further investigated the regulatory effects of andrographolide on platelet apoptotic events. In this study, apoptotic signaling events for caspase-3, -8, and Bid were time (10-60 min)- and dose (25-100 μΜ)-dependently activated by andrographolide in human platelets. Andrographolide could also disrupt mitrochondrial membrane potential. In addition, caspase-8 inhibitor (z-IETD-fmk, 50 μΜ) was found to reverse andrographolide-induced caspase-8 activation, whereas the antagonistic anti-Fas receptor (ZB4, 500 ng/mL) and anti-tumor necrosis factor-R1 (H398, 10 µg/mL) monoclonal antibodies did not. In conclusion, this study for the first time demonstrated that andrographolide might limit platelet lifespan by initiating the caspase-8-dependent extrinsic apoptotic pathway, in spite of no direct evidence that death receptors are involved in this process proved. Overall, the various medicinal properties of andrographolide suggest its potential value in treating patients with thromboembolic disorders.

Activation of caspases-9, -3 and -8 in human platelets triggered by BH3-only mimetic ABT-737 and calcium ionophore A23187: caspase-8 is activated via bypass of the death receptors

Platelet apoptosis and activation have been studied in human platelets treated with BH3-only mimetic ABT-737 and calcium ionophore A23187, agents triggering apoptosis through the intrinsic mitochondrial pathway. Platelet apoptosis was determined as activation of crucial apoptosis-associated caspases, initiator caspase-9 of intrinsic apoptosis pathway, executioner caspase-3 and initiator caspase-8 of extrinsic death receptor pathway, and platelet activation was detected by P-selectin (CD62) exposure on the platelet surface. We found that ABT-737 predominantly induced activation of caspases-9, -3 and -8 rather than CD62 exposure, whereas A23187 induces both caspases activation and CD62 exposure. Caspase-8 activation was stimulated independently of the extrinsic apoptosis pathway via mitochondrial membrane permeabilization and depolarization. These data suggest that (i) caspase-8 activation is triggered in ABT-737- and A23187-treated anucleate platelets through the mitochondria-initiated caspase activation cascade bypassing the death receptors, and (ii) ABT-737-treated platelets are a useful experimental tool for discerning the role of platelet apoptosis in platelet function and survival.

Cisplatin induces platelet apoptosis through the ERK signaling pathway

Cisplatin (cis-diamminedichloroplatinum II) is one of the most widely used anti-tumor agents. However, cisplatin-based chemotherapy is usually accompanied by adverse side effects such as thrombocytopenia, and the mechanism remains unclear. Here we show that cisplatin induced several platelet apoptotic events including up-regulation of Bax and Bak, down-regulation of Bcl-2 and Bcl-X(L), mitochondrial translocation of Bax, mitochondrial inner transmembrane potential depolarization, caspase-3 activation and phosphatidylserine (PS) exposure. Cisplatin dose-dependently induced activation of extracellular signal-regulated protein kinase (ERK) in platelets. Caspase-3 inhibitor z-DEVD-fmk dramatically inhibited cisplatin-induced caspase-3 activation and PS exposure without affecting ERK activation. Blockade of the ERK pathway significantly prevented platelet apoptosis. Furthermore, levels of reactive oxygen species (ROS) and Ca(2+) were significantly elevated by cisplatin, and scavenging of ROS and Ca(2+) obviously inhibited platelet apoptosis induced by cisplatin. In addition, cisplatin did not induce platelet activation, whereas it obviously impaired platelet functions. These data indicate that cisplatin induces platelet apoptosis through the ERK signaling pathway, which might contribute to cisplatin-related haematological toxicity.

Apoptosis in the anucleate platelet

For many years, programmed cell death, known as apoptosis, was attributed exclusively to nucleated cells. Currently, however, apoptosis is also well-documented in anucleate platelets. This review describes extrinsic and intrinsic pathways of apoptosis in nucleated cells and in platelets, platelet apoptosis induced by multiple chemical stimuli and shear stresses, markers of platelet apoptosis, mitochodrial control of platelet apoptosis, and apoptosis mediated by platelet surface receptors PAR-1, GPIIbIIIa and GPIbα. In addition, this review presents data on platelet apoptosis provoked by aging of platelets in vitro during platelet storage, platelet apoptosis in pathological settings in humans and animal models, and inhibition of platelet apoptosis by cyclosporin A, intravenous immunoglobulin and GPIIbIIIa antagonist drugs.

In vitro quality of single-donor platelets treated with riboflavin and ultraviolet light and stored in platelet storage medium for up to 8 days

BACKGROUND

The Mirasol pathogen reduction technology system is known to increase the activation and metabolic rate of platelets (PLTs). Storage of Mirasol PLTs in PLT storage medium (PSM) has the potential to slow this accelerated PLT storage lesion. We investigated the quality of Mirasol-treated PLTs stored in either 50% SSP+ or 50% Composol for 8 days.

STUDY DESIGN AND METHODS

Single-donor double hyperconcentrates were divided between control and Mirasol-treated arms and after treatment were suspended in approximately 50% (vol/vol) SSP+ (n = 8) or Composol (n = 7). In vitro markers of PLT activation and/or apoptosis were measured over an 8-day storage period.

RESULTS

Mirasol treatment resulted in increased spontaneous PLT activation and glycolysis and these effects were worsened when PLTs were treated below a certain volume (150 mL). At higher treatment volumes there were no significant differences between treated units stored in either Composol or SSP+. When low-volume units were stored in Composol the median pH fell below 6.4 on Day 5 and bicarbonate was undetectable, whereas in SSP+ the median pH value was greater than 6.9 and bicarbonate remained at detectable levels, despite other markers of in vitro function being similar to those of Composol.

CONCLUSION

Mirasol treatment of PLTs followed by storage in PSM results in increased PLT activation and metabolism to a level similar to that reported for PLTs treated and stored in plasma. Units treated at a low volume (<150 mL) showed poor in vitro quality.

Mechanostasis in apoptosis and medicine

Mechanostasis describes a complex and dynamic process where cells maintain equilibrium in response to mechanical forces. Normal physiological loading modes and magnitudes contribute to cell proliferation, tissue growth, differentiation and development. However, cell responses to abnormal forces include compensatory apoptotic mechanisms that may contribute to the development of tissue disease and pathological conditions. Mechanotransduction mechanisms tightly regulate the cell response through discrete signaling pathways. Here, we provide an overview of links between pro- and anti-apoptotic signaling and mechanotransduction signaling pathways, and identify potential clinical applications for treatments of disease by exploiting mechanically-linked apoptotic pathways.

Calpain activator dibucaine induces platelet apoptosis

Calcium-dependent calpains are a family of cysteine proteases that have been demonstrated to play key roles in both platelet glycoprotein Ibα shedding and platelet activation and altered calpain activity is associated with thrombotic thrombocytopenic purpura. Calpain activators induce apoptosis in several types of nucleated cells. However, it is not clear whether calpain activators induce platelet apoptosis. Here we show that the calpain activator dibucaine induced several platelet apoptotic events including depolarization of the mitochondrial inner transmembrane potential, up-regulation of Bax and Bak, down-regulation of Bcl-2 and Bcl-X_L, caspase-3 activation and phosphatidylserine exposure. Platelet apoptosis elicited by dibucaine was not affected by the broad spectrum metalloproteinase inhibitor GM6001. Furthermore, dibucaine did not induce platelet activation as detected by P-selectin expression and PAC-1 binding. However, platelet aggregation induced by ristocetin or α-thrombin, platelet adhesion and spreading on von Willebrand factor were significantly inhibited in platelets treated with dibucaine. Taken together, these data indicate that dibucaine induces platelet apoptosis and platelet dysfunction.

Platelet apoptosis by cold-induced glycoprotein Ibα clustering

BACKGROUND

Cold-storage of platelets followed by rewarming induces changes in Glycoprotein (GP) Ibα-distribution indicative of receptor clustering and initiates thromboxane A(2) -formation. GPIbα is associated with 14-3-3 proteins, which contribute to GPIbα-signaling and in nucleated cells take part in apoptosis regulation.

OBJECTIVES AND METHODS

We investigated whether GPIbα-clustering induces platelet apoptosis through 14-3-3 proteins during cold (4 h 0 °C)-rewarming (1 h 37 °C).

RESULTS

During cold-rewarming, 14-3-3 proteins associate with GPIbα and dissociate from Bad inducing Bad-dephosphorylation and activation. This initiates pro-apoptosis changes in Bax/Bcl-x(L) and Bax-translocation to the mitochondria, inducing cytochrome c release. The result is activation of caspase-9, which triggers phosphatidylserine exposure and platelet phagocytosis by macrophages. Responses are prevented by N-acetyl-D-glucosamine (GN), which blocks GPIbα-clustering, and by O-sialoglycoprotein endopeptidase, which removes extracellular GPIbα.

CONCLUSIONS

Cold-rewarming triggers apoptosis through a GN-sensitive GPIbα-change indicative of receptor clustering. Attempts to improve platelet transfusion by cold-storage should focus on prevention of the GPIbα-change.

Platelet apoptosis and activation in platelet concentrates stored for up to 12 days in plasma or additive solution

BACKGROUND

Several studies suggest that apoptosis of platelets occurs during storage of platelet concentrates (PC). We sought to determine whether storage of PC in additive solution alters levels of apoptosis during storage beyond the current shelf life (5-7 days).

STUDY DESIGN AND METHODS

Pooled buffy coat PC (n = 7) were prepared in either 100% plasma or 70% Composol and stored at 22 °C for 12 days. A third arm of the study stored PC in 100% plasma at 37 °C, which is thought to induce apoptosis. PC were tested for mitochrondrial membrane potential, annexin V binding, microparticles, caspase-3/7 activity and decoy cell death receptor 2, as well as standard platelet quality tests.

RESULTS

Composol units remained ≥pH 6·88, with 36% lower lactate and higher pH vs plasma by day 12 (P < 0·001). Platelet function was better maintained, and activation and apoptotic markers tended to be lower in Composol units towards the end of storage. However, levels of all apoptosis markers assessed were not significantly different in units stored in Composol. Storage at 37 °C saw stronger correlation of apoptotic markers with standard quality tests compared to 22 °C, but loss of correlation of caspase-3/7 activity with other apoptosis markers.

CONCLUSION

We conclude that storage of platelets in 70% Composol vs 100% plasma does not increase the rate of platelet apoptosis. Our data agree with other studies suggesting that platelet apoptosis is sequential to high levels of activation, but share a significant degree of overlap.

Phosphatidylserine exposure and other apoptotic-like events in Bernard-Soulier syndrome platelets

In the Bernard-Soulier syndrome (BSS), the giant platelets are said to have increased phosphatidylserine (PS) surface exposure in the resting state and shortened survival in the circulation. When normal platelets are activated, they undergo many biochemical and morphological changes, some of which are apoptotic. Herein, we investigated apoptotic-like events in BSS platelets upon activation, specifically, PS exposure, microparticle (MP) formation, cell shrinkage, and loss of mitochondrial inner membrane potential (DeltaPsi(m)). Platelets from two unrelated BSS patients were examined in whole blood; agonists used were collagen, thrombin, PAR1- or PAR4-activating peptides (APs), or combinations of collagen with thrombin, and the PAR-APs. Flow cytometry was used to measure PS exposure (annexin A5 binding), platelet-derived MPs (forward scatter; events <0.75 microm size), and DeltaPsi(m) (TMRM fluorescence). PS exposure was increased on resting and activated BSS platelets, and this was independent of the platelet size. MP formation by BSS platelets was generally enhanced. Cell shrinkage occurred on activation to form smaller, PS-exposing platelets in BSS and controls. A proportion of PS-exposing BSS and control platelets exhibited DeltaPsi(m) loss, but unlike controls, there was also loss of DeltaPsi(m) in the BSS platelets not exposing PS. Thus, BSS platelets undergo apoptotic-like events upon activation, with PS exposure and MP formation being enhanced. These events may play a role in the shortened survival in BSS, as well as affecting thrombin generation.

Device Thrombogenicity Emulator (DTE)--design optimization methodology for cardiovascular devices: a study in two bileaflet MHV designs

Patients who receive prosthetic heart valve (PHV) implants require mandatory anticoagulation medication after implantation due to the thrombogenic potential of the valve. Optimization of PHV designs may facilitate reduction of flow-induced thrombogenicity and reduce or eliminate the need for post-implant anticoagulants. We present a methodology entitled Device Thrombogenicty Emulator (DTE) for optimizing the thrombo-resistance performance of PHV by combining numerical and experimental approaches. Two bileaflet mechanical heart valves (MHV) designs, St. Jude Medical (SJM) and ATS, were investigated by studying the effect of distinct flow phases on platelet activation. Transient turbulent and direct numerical simulations (DNS) were conducted, and stress loading histories experienced by the platelets were calculated along flow trajectories. The numerical simulations indicated distinct design dependent differences between the two valves. The stress loading waveforms extracted from the numerical simulations were programmed into a hemodynamic shearing device (HSD), emulating the flow conditions past the valves in distinct 'hot-spot' flow regions that are implicated in MHV thrombogenicity. The resultant platelet activity was measured with a modified prothrombinase assay, and was found to be significantly higher in the SJM valve, mostly during the regurgitation phase. The experimental results were in excellent agreement with the calculated platelet activation potential. This establishes the utility of the DTE methodology for serving as a test bed for evaluating design modifications for achieving better thrombogenic performance for such devices.

High-shear stress sensitizes platelets to subsequent low-shear conditions

Individuals with mechanical heart valve implants are plagued by flow-induced thromboembolic complications, which are undoubtedly caused by platelet activation. Flow fields in or around the affected regions involve brief exposure to pathologically high-shear stresses on the order of 100 to 1000 dyne/cm(2). Although high shear is known to activate platelets directly, their subsequent behavior is not known. We hypothesize that the post-high-shear activation behavior of platelets is particularly relevant in understanding the increased thrombotic risk associated with blood-recirculating prosthetic cardiovascular devices. Purified platelets were exposed to brief (5-40 s) periods of high-shear stress, and then exposed to longer periods (15-60 min) of low shear. Their activation state was measured using a prothrombinase-based assay. Platelets briefly exposed to an initial high-shear stress (e.g., 60 dyne/cm(2) for 40 s) activate a little, but this study shows that they are now sensitized, and when exposed to subsequent low shear stress, they activate at least 20-fold faster than platelets not initially exposed to high shear. The results show that platelets in vitro exposed beyond a threshold of high-shear stress are primed for subsequent activation under normal cardiovascular circulation conditions, and they do not recover from the initial high-shear insult.

Comparison of the relative activities of inducing platelet apoptosis stimulated by various platelet-activating agents

Apoptosis-like events are known to occur in anuclear platelets. Although the mechanisms responsible for these events are still not completely understood, studies suggested that some platelet agonists can activate platelet apoptosis. However, the relative activities of various platelet agonists in inducing apoptosis have not yet been investigated. In the present study we explored this issue, and attempted to identify the correlation between platelet activation and apoptosis. In a platelet aggregation study, there were no significant differences respectively stimulated by arachidonic acid (AA; 100 microM), ADP (20 microM), collagen (10 microg/mL), thrombin (0.1 U/mL), U46619 (10 microM), and A23187 (5 microM). In a subsequent study, we fixed these concentrations of agonists to further compare their relative activities in inducing platelet apoptosis. Our results found that thrombin, U46619, and A23187 possess stronger activities than the other agonists in inducing platelet apoptosis (i.e., phosphatidylserine exposure, mitochondrial membrane potential depolarization, eukaryotic initiation factor (eIF)2alpha, and caspase activation). On the other hand, AA induced no apoptotic events in platelets. Based on this approach, we demonstrated for the first time that thrombin, U46619, and A23187, but not AA, possess stronger activity in inducing platelet apoptosis. In addition, we also found that platelet activation might not necessarily be associated with the occurrence of platelet apoptosis. The in vivo physiological function of the apoptotic machinery in platelets is not yet clearly understood, and needs to be further investigated in the future.

The glycoprotein Ibalpha-von Willebrand factor interaction induces platelet apoptosis

BACKGROUND

The interaction of glycoprotein (GP) Ibalpha with von Willebrand factor (VWF) initiates platelet adhesion, and simultaneously triggers intracellular signaling cascades leading to platelet aggregation and thrombus formation. Some of the signaling events are similar to those occurring during apoptosis, however, it is still unclear whether platelet apoptosis is induced by the GPIbalpha-VWF interaction.

OBJECTIVES

To investigate whether the GPIbalpha-VWF interaction induces platelet apoptosis and the role of 14-3-3zeta in apoptotic signaling.

METHODS

Apoptotic events were assessed in platelets or Chinese hamster ovary (CHO) cells expressing wild-type (1b9) or mutant GPIb-IX interacting with VWF by flow cytometry or western blotting.

RESULTS

Ristocetin-induced GPIbalpha-VWF interaction elicited apoptotic events in platelets, including phosphatidylserine exposure, elevations of Bax and Bak, gelsolin cleavage, and depolarization of mitochondrial inner transmembrane potential. Apoptotic events were also elicited in platelets exposed to pathologic shear stresses in the presence of VWF; however, the shear-induced apoptosis was eliminated by the anti-GPIbalpha antibody AK2. Furthermore, apoptotic events occurred in 1b9 cells stimulated with VWF and ristocetin, but were significantly diminished in two CHO cell lines expressing mutant GPIb-IX with GPIbalpha truncated at residue 551 or a serine-to-alanine mutation at the 14-3-3zeta-binding site in GPIbalpha.

CONCLUSIONS

This study demonstrates that the GPIbalpha-VWF interaction induces apoptotic events in platelets, and that the association of 14-3-3zeta with the cytoplasmic domain of GPIbalpha is essential for apoptotic signaling. This finding may suggest a novel mechanism for platelet clearance or some thrombocytopenic diseases.

Shear-induced interaction of platelets with von Willebrand factor results in glycoprotein Ibα shedding

Shear-induced platelet adhesion through the interaction of glycoprotein (GP) Ibα with von Willebrand factor (VWF) exposed at the injured vessel wall or atherosclerotic plaque rupture is a prerequisite for the physiological hemostatic process or pathological thrombus formation in stenosed arteries. Here we show that shear-induced interaction of platelets with immobilized VWF results in GPIbα ectodomain shedding. Washed platelets were exposed to VWF-coated glass capillary or cone-and-plate viscometer at different shear rates, and GPIbα ectodomain was shed from platelets, while a small mass of GPIbα COOH-terminal peptide, ∼17 kDa, was increased correspondingly. The extent of GPIbα shedding was enhanced with the concentration of immobilized VWF and the time duration of constant shear stress, whereas it was obviously reduced with the decreased number of adherent platelets. Pretreatment of platelets with membrane-permeable calpain inhibitors and metalloproteinase inhibitor abolished shear-induced GPIbα shedding. Furthermore, GPIbα shedding was obviously diminished by anti-integrin-αIIbβ3monoclonal antibody SZ21, phosphatidylinositol 3-kinase inhibitor wortmannin, and cell-permeable calcium chelator 1,2-bis( o-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid. These results indicate that shear-induced platelet-VWF interaction results in calpain and metalloproteinase-dependent GPIbα ectodomain shedding. These findings not only have a physiological implication in understanding the presence of glycocalicin in normal circulation, but also suggest a novel mechanism for the negative regulation of platelet function and the limitation of platelet thrombus infinite formation under pathophysiological flow conditions.

The role of the intrinsic apoptosis pathway in platelet life and death

In recent years, it has become increasingly apparent that the production of platelets and their subsequent life span in the circulation are regulated, at least in part, by apoptotic mechanisms. There is also evidence implicating the apoptotic machinery in the regulation of platelet functional responses. This review examines the role of the intrinsic apoptosis pathway, regulated by the Bcl-2 family of proteins, in platelet biology.

Mitochondrial control of platelet apoptosis: effect of cyclosporin A, an inhibitor of the mitochondrial permeability transition pore

The role of the mitochondrial permeability transition pore (MPTP) in apoptosis of nucleated cells is well documented. In contrast, the role of MPTP in apoptosis of anucleated platelets is largely unknown. The aim of this study was to elucidate the contribution of MPTP in the control of different manifestations of platelet apoptosis by analyzing the effect of cyclosporin A (CsA), a potent inhibitor of MPTP formation. Using flow cytometry, we studied the effect of pretreatment of platelets with CsA on apoptotic responses in human platelets stimulated with calcium ionophore A23187. We found that CsA inhibited A23187-stimulated platelet apoptosis, completely preventing (i) depolarization of mitochondrial inner membrane potential (DeltaPsim), (ii) activation of cytosolic apoptosis executioner caspase-3, (iii) platelet shrinkage, and (iv) fragmentation of platelets to microparticles, but (v) only partially (approximately 25%), inhibiting phosphatidylserine (PS) exposure on the platelet surface. This study shows that MPTP formation is upstream of DeltaPsim depolarization, caspase-3 activation, platelet shrinkage and microparticle formation, and stringently controls these apoptotic events in A23187-stimulated platelets but is less involved in PS externalization. These data also indicate that CsA may rescue platelets from apoptosis, preventing caspase-3 activation and inhibiting the terminal cellular manifestations of platelet apoptosis, such as platelet shrinkage and degradation to microparticles. Furthermore, the results suggest a novel potentially useful application of CsA as an inhibitor of platelet demise through apoptosis in thrombocytopenias associated with enhanced platelet apoptosis.

The uptake of apoptotic cells drives Coxiella burnetii replication and macrophage polarization: a model for Q fever endocarditis

Patients with valvulopathy have the highest risk to develop infective endocarditis (IE), although the relationship between valvulopathy and IE is not clearly understood. Q fever endocarditis, an IE due to Coxiella burnetii, is accompanied by immune impairment. Patients with valvulopathy exhibited increased levels of circulating apoptotic leukocytes, as determined by the measurement of active caspases and nucleosome determination. The binding of apoptotic cells to monocytes and macrophages, the hosts of C. burnetii, may be responsible for the immune impairment observed in Q fever endocarditis. Apoptotic lymphocytes (AL) increased C. burnetii replication in monocytes and monocyte-derived macrophages in a cell-contact dependent manner, as determined by quantitative PCR and immunofluorescence. AL binding induced a M2 program in monocytes and macrophages stimulated with C. burnetii as determined by a cDNA chip containing 440 arrayed sequences and functional tests, but this program was in part different in monocytes and macrophages. While monocytes that had bound AL released high levels of IL-10 and IL-6, low levels of TNF and increased CD14 expression, macrophages that had bound AL released high levels of TGF-beta1 and expressed mannose receptor. The neutralization of IL-10 and TGF-beta1 prevented the replication of C. burnetii due to the binding of AL, suggesting that they were critically involved in bacterial replication. In contrast, the binding of necrotic cells to monocytes and macrophages led to C. burnetii killing and typical M1 polarization. Finally, interferon-gamma corrected the immune deactivation induced by apoptotic cells: it prevented the replication of C. burnetii and re-directed monocytes and macrophages toward a M1 program, which was deleterious for C. burnetii. We suggest that leukocyte apoptosis associated with valvulopathy may be critical for the pathogenesis of Q fever endocarditis by deactivating immune cells and creating a favorable environment for bacterial persistence.

Biological effects of dynamic shear stress in cardiovascular pathologies and devices

Altered and highly dynamic shear stress conditions have been implicated in endothelial dysfunction leading to cardiovascular disease, and in thromboembolic complications in prosthetic cardiovascular devices. In addition to vascular damage, the pathological flow patterns characterizing cardiovascular pathologies and blood flow in prosthetic devices induce shear activation and damage to blood constituents. Investigation of the specific and accentuated effects of such flow-induced perturbations on individual cell-types in vitro is critical for the optimization of device design, whereby specific design modifications can be made to minimize such perturbations. Such effects are also critical in understanding the development of cardiovascular disease. This review addresses limitations to replicate such dynamic flow conditions in vitro and also introduces the idea of modified in vitro devices, one of which is developed in the authors' laboratory, with dynamic capabilities to investigate the aforementioned effects in greater detail.

Critical role for the mitochondrial permeability transition pore and cyclophilin D in platelet activation and thrombosis

Many of the cellular responses that occur in activated platelets resemble events that take place following activation of cell-death pathways in nucleated cells. We tested the hypothesis that formation of the mitochondrial permeability transition pore (MPTP), a key signaling event during cell death, also plays a critical role in platelet activation. Stimulation of murine platelets with thrombin plus the glycoprotein VI agonist convulxin resulted in a rapid loss of mitochondrial transmembrane potential (Deltapsi(m)) in a subpopulation of activated platelets. In the absence of cyclophilin D (CypD), an essential regulator of MPTP formation, murine platelet activation responses were altered. CypD-deficient platelets exhibited defects in phosphatidylserine externalization, high-level surface fibrinogen retention, membrane vesiculation, and procoagulant activity. Also, in CypD-deficient platelet-rich plasma, clot retraction was altered. Stimulation with thrombin plus H(2)O(2), a known activator of MPTP formation, also increased high-level surface fibrinogen retention, phosphatidylserine externalization, and platelet procoagulant activity in a CypD-dependent manner. In a model of carotid artery photochemical injury, thrombosis was markedly accelerated in CypD-deficient mice. These results implicate CypD and the MPTP as critical regulators of platelet activation and suggest a novel CypD-dependent negative-feedback mechanism regulating arterial thrombosis.

Flow-induced Platelet Activation and Damage Accumulation in a Mechanical Heart Valve: Numerical Studies

A model for platelet activation based on the theory of damage, incorporating cumulative effects of stress history and past damage (senescence) was applied to a three-dimensional (3-D) model of blood flow through a St. Jude Medical (SJM) bileaflet mechanical heart valve (MHV), simulating flow conditions after implantation. The calculations used unsteady Reynolds-averaged Navier-Stokes formulation with non-Newtonian blood properties. The results were used to predict platelet damage from total stress (shear, turbulent, deformation), and incorporate the contribution of repeated passages of the platelets along pertinent trajectories. Trajectories that exposed the platelets to elevated levels of stress around the MHV leaflets and led them to entrapment within the complex 3-D vortical structures in the wake of the valve significantly enhanced platelet activation. This damage accumulation model can be used to quantify the thrombogenic potential of implantable cardiovascular devices, and indicate the problem areas of the device for improving their designs.

Low-intensity ultrasound inhibits apoptosis and enhances viability of human mesenchymal stem cells in three-dimensional alginate culture during chondrogenic differentiation

Many studies have investigated optimal chondrogenic conditions, but only a few of them have addressed their effects on cell viability or the methods to enhance it. This study investigated the effect of low-intensity ultrasound (LIUS), a well-known chondrogenic inducer, on the viability of human mesenchymal stem cells (hMSCs) during chondrogenic differentiation in three-dimensional (3-D) alginate culture. The hMSCs/alginate layer was cultured in a chondrogenic defined medium and treated with transforming growth factor-beta1 (TGF-beta1) and/or LIUS for 2 weeks. Along with chondrogenic differentiation for 2 weeks, the 3-D alginate culture and TGF-beta1 treatment resulted in the decrease of cell viability, which appeared to be mediated by apoptosis. In contrast, co-treatment with LIUS clearly enhanced cell viability and inhibited apoptosis under the same conditions. The effect of LIUS on the apoptotic event was further demonstrated by changes in the expression of apoptosis/viability related genes of p53, bax, bcl-2, and PCNA. These results suggest that the LIUS treatment could be a valuable tool in cartilage tissue engineering using MSCs as it enhances cell viability and directs the chondrogenic differentiation process, its well-known activity.

Higher thrombin concentrations are required to induce platelet apoptosis than to induce platelet activation

Primarily known as an inducer of blood coagulation and platelet activation, thrombin also triggers platelet apoptosis. This study demonstrated that the platelet activation response is much more sensitive than platelet apoptosis to thrombin treatment. Thrombin concentrations of 0.5-1 nmol/l activated almost all platelets, but only a small fraction underwent apoptosis, suggesting that at these relatively low thrombin concentrations, platelets may perform haemostasis but not be involved in programmed cell death. At high thrombin concentrations of 10-100 nmol/l, generated during blood coagulation, 30-40% of platelets became apoptotic, indicating that hypercoagulable states may be associated with increased numbers of apoptotic platelets.

Persistence of procoagulant surface expression on activated human platelets: involvement of apoptosis and aminophospholipid translocase activity

BACKGROUND

Activated platelets express a procoagulant surface when the asymmetric distribution of membrane phospholipids is scrambled, leading to phosphatidylserine (PS) exposure. PS expression, associated with apoptosis in nucleated cells, would be expected to be reversed by aminophospholipid translocase (APLT) activity.

OBJECTIVE

To determine whether the procoagulant surface of activated platelets persists after it forms; to examine whether PS expression on platelets is associated with loss of mitochondrial inner membrane potential (DeltaPsi(m)), a hallmark of apoptosis; and to investigate the role of APLT in persistence of PS expression.

METHODS

Platelets were stimulated with thrombin, collagen, a combination of both, or the Ca(2+)-ionophore A23187. Up to 4 h after activation, procoagulant surface expression was measured by annexin A5 binding by flow cytometry and by a prothrombinase assay. Flow cytometry was also used to measure PS expression concurrently with DeltaPsi(m) collapse, using CMXRos. APLT activity in annexin A5-negative and -positive platelets was measured flow cytometrically as the percent of 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphatidylserine (NBD-PS) translocated from the outer to the inner membrane leaflet.

RESULTS AND CONCLUSIONS

Procoagulant surface expression on activated platelets persisted in vitro for at least 4 h; if such persistence occurs in vivo, there are important implications for the propagation of thrombosis. With the physiological stimuli, only 10-20% of the activated platelets expressed PS on their surface, and of these, only a portion exhibited DeltaPsi(m) collapse, indicating that PS expression can be associated with platelet apoptosis, but can also occur independently. APLT activity was very low in the PS-expressing platelet subpopulation for up to 4 h after activation, indicating that the persistence of a procoagulant surface may be attributed, at least in part, to this reduced APLT activity.

Cinnamtannin B-1 from bay wood exhibits antiapoptotic effects in human platelets

Proanthocyanidins, such as cinnamtannin B-1, are polyphenolic compounds with antioxidant activity that induce apoptosis in a number of tumoral cells. We have now investigated the pro- or anti-apoptotic effects of cinnamtannin B-1 in human platelets. Platelet stimulation with thrombin induced cellular apoptosis, as detected by phosphatidylserine exposure and the activation of caspases-3 and -9. Pretreatment for 30 min with cinnamtannin B-1 impaired thrombin-induced apoptosis in platelets. Thrombin has been shown to induce H(2)O(2) generation in platelets, which induced similar apoptotic events than thrombin in these cells. Pretreatment with cinnamtannin B-1 reduced H(2)O(2)-induced phosphatidylserine exposure and caspase activation. Finally, platelet stimulation with thrombin induced translocation of caspases-3 and -9 to the cytoskeletal (Triton-insoluble) fraction, which is important for their activation and the development of apoptotic events. Pretreatment with cinnamtannin B-1 impaired translocation of caspases-3 and -9 to the cytoskeleton and, as a result, procaspases are accumulated in the Triton-soluble fraction. Our results provide evidence for the antiapoptotic actions of cinnamtannin B-1 in human platelets.

Thrombin-triggered platelet apoptosis

BACKGROUND

Thrombin is primarily known as a coagulation factor and as an inducer of platelet activation and aggregation. It has been reported that thrombin modulates apoptosis of nucleated cells.

OBJECTIVES

The current study investigated whether thrombin can affect apoptosis in anucleated human platelets.

METHODS

Using flow cytometry, we studied platelet apoptosis at the single-cell level, analyzing markers of mitochondrial and cytoplasmic apoptosis. Western blotting was also employed, in addition to flow cytometry, for determining the expression of Bcl-2 family proteins.

RESULTS

We found that human alpha-thrombin induced four key manifestations of apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (DeltaPsi m) depolarization; (ii) strong expression of pro-apoptotic Bax and Bak proteins but only weak expression of anti-apoptotic Bcl-2 protein; (iii) caspase-3 activation; and (iv) phosphatidylserine (PS) exposure.

CONCLUSIONS

This study demonstrates that, aside from its 'classical' function as an inducer of platelet activation, thrombin can trigger platelet apoptosis, where it acts as a death ligand. These data indicate that thrombin triggers platelet apoptosis by impacting on several intracellular apoptotic targets, including shifting the balance between Bcl-2 regulatory proteins in a pro-apoptotic direction, depolarizing the inner mitochondrial membrane, activating the executioner caspase-3, and stimulating aberrant exposure of PS on the platelet surface.

Caspases 3 and 9 are translocated to the cytoskeleton and activated by thrombin in human platelets. Evidence for the involvement of PKC and the actin filament polymerization

Platelets express, among others, initiator caspase 9 and effector caspase 3. Upon activation by physiological agonists, calcium ionophores or under shear stress they might develop apoptotic events. Although it is well known that the cytoskeletal network plays a crucial role in apoptosis, the relationship between caspases 3 and 9 and the cytoskeleton is poorly understood. Here we demonstrate that the physiological agonist thrombin is able to induce activation of caspases 3 and 9 in human platelets and significantly increases the amount in the cytoskeleton of the active forms of both caspases and the procaspases 3 and 9. After stimulation with thrombin the amount of active caspases 3 and 9 in the cytosolic and cytoskeletal fractions were significantly reduced in Ro-31-8220-treated cells, which demonstrates that caspases activation and association with the cytoskeleton needs the contribution of PKC. Inhibition of actin polymerization by cytochalasin D inhibits translocation and activation of both caspases, suggesting that thrombin stimulates caspase 3 and 9 activation and association with the reorganizing actin cytoskeleton. Finally, our results show that inhibition of thrombin-induced caspase activation has no effect on their translocation to the cytoskeleton although impairment of thrombin-evoked caspase translocation has negative effects on caspase activity, suggesting that translocation to the cytoskeleton might be important for caspase activation by thrombin in human platelets.

Shear Stress Induces Preimplantation Embryo Death That Is Delayed by the Zona Pellucida and Associated with Stress-Activated Protein Kinase-Mediated Apoptosis1

In this study, we discovered that embryos sense shear stress and sought to characterize the kinetics and the enzymatic mechanisms underlying induction of embryonic lethality by shear stress. Using a rotating wall vessel programmed to produce 1.2 dynes/cm2 shear stress, it was found that shear stress caused lethality within 12 h for E3.5 blastocysts. Embryos developed an approximate 100% increase in mitogen-activated protein kinase 8/9 (formerly known as stress-activated protein kinase/junC kinase 1/2) phosphorylation by 6 h of shear stress that further increased to approximately 350% by 12 h. Terminal deoxynucleotidyltransferase dUTP nick end labeling/apoptosis was at baseline levels at 6 h and increased to approximately 500% of baseline at 12 h, when irreversible commitment to death occurred. A mitogen-activated protein kinase 8/9 phosphorylation inhibitor, D-JNKI1, was able to inhibit over 50% of the apoptosis, suggesting a causal role for mitogen-activated protein kinase 8/9 phosphorylation in the shear stress-induced lethality. The E2.5 (compacted eight-cell/early morula stage) embryo was more sensitive to shear stress than the E3.5 (early blastocyst stage) embryo. Additionally, zona pellucida removal significantly accelerated shear stress-induced lethality while having no lethal effect on embryos in the static control. In conclusion, preimplantation embryos sense shear stress, chronic shear stress is lethal, and the zona pellucida lessens the lethal and sublethal effects of shear stress. Embryos in vivo would not experience as high a sustained velocity or shear stress as induced experimentally here. Lower shear stresses might induce sufficient mitogen-activated protein kinase 8/9 phosphorylation that would slow growth or cause premature differentiation if the zona pellucida were not intact.

Dendritic cells of immune thrombocytopenic purpura (ITP) show increased capacity to present apoptotic platelets to T lymphocytes

OBJECTIVE

Altered self-antigen processing/presentation of apoptotic cells by DCs and/or modifications of autoantigens may lead to the development of autoantibodies. Increasing evidence indicates that platelets may undergo apoptosis. Therefore, in the present study we investigated whether platelet apoptosis and/or dendritic cells (DCs) may play a role in the stimulation of the immuno-mediated anti-platelet response in chronic immune thrombocytopenic purpura (ITP).

METHODS AND RESULTS

Twenty-nine patients with active ITP and 29 healthy adult volunteers were enrolled into the study. Freshly washed platelets and platelets aged in a plasma-free buffer for 72 hours at 37 degrees C were assessed by flow cytometry for phosphatidylserine exposure using annexin V-FITC, caspase activation, and platelet activation markers. CD14-derived DCs were characterized by immunophenotyping, cytokine production, and ability to present fresh and aged platelets to T lymphocytes. We demonstrated that platelets from ITP patients, either fresh or in vitro aged, show increased apoptosis (with low levels of activation) in comparison to their normal counterparts. We also found that immature DCs readily ingest apoptotic platelets. Furthermore, in ITP patients DCs, prepulsed with autologous/allogeneic fresh and aged platelets, are highly efficient in stimulating autologous T-cell proliferation as compared to DCs derived from healthy donors. This finding may be related to the upregulated expression of CD86 in DCs from ITP patients and not to higher phagocytic activity.

CONCLUSION

These results suggest that DC dysfunction, together with increased propensity of platelets to undergo apoptosis, may play a role in the stimulation of the immune system in ITP.