Early increase in DcR2 expression and late activation of caspases in the platelet storage lesion

Platelet intrinsic apoptosis

In a healthy individual, the lifespan of most platelets is tightly regulated by intrinsic, or mitochondrial, apoptosis. This is a special form of programmed cell death governed by the BCL-2 family of proteins, where the prosurvival protein BCL-XL maintains platelet viability by restraining the prodeath proteins BAK and BAX. Restriction of platelet lifespan by activation of BAK and BAX mediated intrinsic apoptosis is essential to maintain a functional, haemostatically reactive platelet population. This review focuses on the molecular regulation of intrinsic apoptosis in platelets, reviews conditions linked to enhanced platelet death, discusses ex vivo storage of platelets and describes caveats associated with the assessment of platelet apoptosis.

CPT2 K79 Acetylation Regulates Platelet Lifespan

CPT2 K79 acetylation caused by NAD⁺ exhaustion and Sirt3 dysfunction resulted in LCAC accumulation and platelet damage.

Blocking acylcarnitine generation with AMPK or CPT1 inhibitors, Sirt3 agonists, and antioxidants retarded platelet storage lesion.

The short life span of platelets is a major challenge to platelet transfusion services because of the lack of effective intervention. Here, we found that the accumulation of long-chain acylcarnitines (LCACs) is responsible for mitochondrial damage and platelet storage lesion. Further studies showed that the blockade of fatty acid oxidation and the activation of AMP-activated protein kinase (AMPK)/acetyl-CoA carboxylase/carnitine palmitoyltransferase 1 (CPT1) pathways that promote fatty acid metabolism are important reasons for the accumulation of LCACs. The excessive accumulation of LCACs can cause mitochondrial damage and a short life span of stored platelets. The mechanism study elucidated that NAD⁺ exhaustion and the subsequent decrease in sirtuin 3 (Sirt3) activity caused an increase in the level of CPT2 K79 acetylation, which is the primary cause of the blockade of fatty acid oxidation and the accumulation of LCACs. Blocking LCAC generation with the inhibitors of AMPK or CPT1, the agonists of Sirt3, and antioxidants tremendously retarded platelet storage lesion in vitro and prolonged the survival of stored platelets in vivo posttransfusion with single or combined use. In summary, we discovered that CPT2 acetylation attenuates fatty acid oxidation and exacerbates platelet storage lesion and may serve as a new target for improving platelet storage quality.

Platelets express adaptor proteins of the extrinsic apoptosis pathway and can activate caspase-8

Background

Apoptotic pathways in platelets are important for their survival and function. Platelet apoptosis may be involved in the pathogenesis of immune thrombocytopenia (ITP), an autoimmune-mediated disease. In contrast to the intrinsic apoptosis pathway, not much is known about the extrinsic pathway mechanisms in platelets.

Objectives

To investigate the expression of proteins involved in the extrinsic apoptosis pathway, including the death receptors, adaptor and regulator proteins in human platelets. To determine a possible trigger of the extrinsic apoptosis pathway in platelets.

Methods

To investigate the expression of key markers of the extrinsic pathway we used targeted immunofluorescence and flow cytometry assays. To study their expression and interaction we performed Western blotting and co-immunoprecipitation. Treated platelets with different apoptosis triggers were subjected to flow cytometry.

Results

We could identify the protein expression of the pro-apoptotic proteins TRADD (Tumor Necrosis Factor Receptor type 1- Associated DEATH Domain protein), TRAF2/5, (TNF Associated Factor) and DEDAF (Death Effector Domain- Associated Factor), FADD (Fas-Associated protein with death domain) as well as the anti-apoptotic proteins DJ-1 (Deglycase 1) and c-FLIP in human platelets. ABT-737 treatment induced a disruption in the co-localization of DJ-1 with FADD. Platelets treated with ABT-737 showed an activation in caspase-3 and -8. The exposure to TNF (Tumor Necrosis Factor), FasL (Fas ligand), and TWEAK or to plasma derived from ITP patients, did not lead to changes in caspase-3 and -8 activation in platelets.

Conclusions

Human platelets express some proteins of the extrinsic apoptosis pathway which can be modulated only by ABT-737 treatment. However so far, no other apoptosis trigger or interaction with an external receptor have been yet identified.

Treatment of platelet concentrates with the L-carnitine modulates platelets oxidative stress and platelet apoptosis due to mitochondrial reactive oxygen species reduction and reducing cytochrome C release during storage

Platelet concentrate (PC) transfusion is administrated to reduce the hemostatic complications in patients with thrombocytopenia. Strength platelet against oxidative stress conditions lead to decrease in platelet storage lesion (PSL). This study was aimed to evaluate L-carnitine (LC) effects on platelet oxidative stress and platelet apoptosis during storage time. PC bags were randomly selected and each bag was divided into two equal parts. L-carnitine was added to test groups. Normal saline was added to control groups. Platelets count, mean platelet volume (MPV), pH, Platelet aggregation, nitric oxide metabolism (nitric/nitrate), total antioxidant capacity (TAC), malondealdehyde concentration (MDA), lactate dehydrogenase (LDH) enzyme activity, mitochondrial reactive oxygen species (ROS) and cytochrome C releasing were assayed by standard methods in 1, 3, 5 and 7 days of platelet storage. LDH enzyme activity was increased during storage but it had lower level in L-carnitine-treated platelets. LC treatment led to reduction in MDA concentration (3.35 ± 0.98 vs 5.3 ± 1.32, p = 0.003 and 6.52 ± 1.88 vs 5.67 ± 1.25, p = 0.005 for day 5 and day 7 respectively). Increased level of TAC was detected in LC-treated platelets in comparison to control (0.29 ± 0.06 vs 0.21 ± 0.05, p = 0.008 and 0.22 ± 0.03 vs 0.16 ± 0.03, p = 0.003 for day 5 and day 7 respectively). Interestingly, mitochondrial ROS and cytochrome C releasing was significantly lower in LC-treated versus control group during platelet storage. L-carnitine not only decreases mitochondrial ROS but also reduces cytochrome C releasing in PCs during storage. It might be considered as safe additive to decrease PSL in the future.

Protective effect of L-carnitine on platelet apoptosis during storage of platelet concentrate

BACKGROUND

Platelet apoptosis is considered as one of the important factors involved in platelet storage lesion (PSL) and affect the quality of platelets during storage. The beneficial effect of L-carnitine (LC) on platelet apoptosis during platelet concentrates (PCs) storage has not been fully investigated. The aim of this study was to evaluate the effects of LC on platelets of PC regarding their apoptosis markers during storage.

METHODS

Ten PCs from healthy donors were investigated in this study. PCs were prepared by platelet rich plasma (PRP) method and stored at 22±2°C with gentle agitation during storage. The effects of LC (15mM) on the platelet apoptosis were assessed by analyzing different indicative presence or absence of LC. Sampling was performed to evaluate apoptosis markers during platelet storage.

RESULTS

The results indicated significantly higher mitochondrial membrane potential for LC-treated platelets than the untreated on the days 2 and 5 of storage (P_day2=0.001, P_day5=0.001). Phosphatidylserine (PS) exposure significantly increased on the untreated compared with LC-treated platelets on the second and third days of storage (P_day2=0.014, P_day3=0.012). Also, active caspase 3 was lower in the LC- treated platelets than the control group on the day 5 of storage (P_day5=0.004). Cytosolic cytochrome C was so significantly lower in LC-treated compared to the untreated platelets during storage time (P_day2=0.002, P_day3=0.001, Pday5=0.001).

CONCLUSION

The results of this study indicate that the use of LC as an additive solution in platelets may be useful to reduce PSL by decreasing platelet apoptosis via mitochondrial pathway and increase platelet quality during storage.

Effects of storage time prolongation on in vivo and in vitro characteristics of 4°C-stored platelets

BACKGROUND

Cold (4°C)-stored platelets are currently under investigation for transfusion in bleeding patients. It is currently unknown how long cold-stored platelets can be stored for clinical applications.

STUDY DESIGN AND METHODS

Twenty three subjects were recruited. Twenty-one subjects were available for in vivo assessment and received indium-111 radiolabeled, cold-stored platelets. We investigated 5- (n = 5), 10- (n = 6), 15- (n = 5), and 20-day-stored (n = 5) platelets and obtained samples for in vitro testing at baseline and after the designated storage time. Twenty three units were available for in vitro testing. Five- and 7-day (n = 5 each), room temperature (RT)-stored platelets served as the current clinical standard control.

RESULTS

In vivo, we found a continuous decline in platelet recovery from 5 to 20 days. Platelet survival reached a low nadir after 10 days of storage. Ex vivo, we observed the maximum platelet αIIbβ3 integrin response to collagen at 5 days of cold storage, and we saw a continuous decline thereafter. However, platelet integrin activation and mitochondrial membrane integrity were better preserved after 20 days at 4°C, compared to 5 days at RT. Platelet metabolic parameters suggest comparable results between 20-day cold-stored platelets and 5- or 7-day RT-stored platelets.

CONCLUSION

In summary, we performed the first studies with extended, cold-stored, apheresis platelets in plasma for up to 20 days with a fresh comparator. Storing cold-stored platelets up to 20 days yields better results in vitro, but further studies in actively bleeding patients are needed to determine the best compromise between hemostatic efficacy and storage prolongation.

The contribution of oxidative stress to platelet senescence during storage

BACKGROUND

Platelets for transfusion become senescent and dysfunctional during storage, resulting in a markedly short shelf life (5 days). We hypothesized that oxidative stress might account for this decline.

STUDY DESIGN AND METHODS

Human platelets were treated with or without antioxidants before storage, and samples were collected and analyzed at different time points. Platelet senescence was determined by senescence-associated β-galactosidase assay, and senescence-related platelet qualities were also analyzed.

RESULTS

Sign of senescence became evident after Day 3 and continued to increase over time. We also found that chemical induction of platelet activation did not affect senescence level, whereas apoptosis inducers showed a stimulative effect on platelet senescence. Moreover, this effect was not prevented by a pan-caspase inhibitor. Meanwhile, cellular and mitochondrial reactive oxygen species were found elevated during storage, and treatments with antioxidants successfully prevented this increase and also mitigated senescence levels of stored platelets. Finally, resveratrol, a natural antioxidant, was utilized as a novel storage additive to safely extend platelet shelf time. We showed that the addition of resveratrol efficiently postponed platelet senescence and ameliorated platelet storage lesion.

CONCLUSIONS

Platelets during storage became senescent and dysfunctional over time, and we found that oxidative stress might account for this decline. The addition of antioxidants effectively postponed senescence and ameliorated platelet storage lesion, which might provide a valuable reference to future platelet storage methodologies.

Intrinsic apoptosis circumvents the functional decline of circulating platelets but does not cause the storage lesion

BAK/BAX depletion in murine platelets reveals that intrinsic apoptosis is not required for the development of the platelet storage lesion. Restriction of platelet life span by intrinsic apoptosis is pivotal to maintain a functional, hemostatically reactive platelet population.

Platelet Storage Lesions: What More Do We Know Now?

Platelet concentrate (PC) transfusions are a lifesaving adjunct to control and prevent bleeding in cancer, hematologic, surgical, and trauma patients. Platelet concentrate availability and safety are limited by the development of platelet storage lesions (PSLs) and risk of bacterial contamination. Platelet storage lesions are a series of biochemical, structural, and functional changes that occur from blood collection to transfusion. Understanding of PSLs is key for devising interventions that prolong PC shelf life to improve PC access and wastage. This article will review advancements in clinical and mechanistic PSL research. In brief, exposure to artificial surfaces and high centrifugation forces during PC preparation initiate PSLs by causing platelet activation, fragmentation, and biochemical release. During room temperature storage, enhanced glycolysis and reduced mitochondrial function lead to glucose depletion, lactate accumulation, and product acidification. Impaired adenosine triphosphate generation reduces platelet capacity to perform energetically demanding processes such as hypotonic stress responses and activation/aggregation. Storage-induced alterations in platelet surface proteins such as thrombin receptors and glycoproteins decrease platelet aggregation. During storage, there is an accumulation of immunoactive proteins such as leukocyte-derive cytokines (tumor necrosis factor α, interleukin (IL) 1α, IL-6, IL-8) and soluble CD40 ligand which can participate in transfusion-related acute lung injury and nonhemolytic transfusion reactions. Storage-induced microparticles have been linked to enhanced platelet aggregation and immune system modulation. Clinically, stored PCs have been correlated with reduced corrected count increment, posttransfusion platelet recovery, and survival across multiple meta-analyses. Fresh PC transfusions have been associated with superior platelet function in vivo; however, these differences were abrogated after a period of circulation. There is currently insufficient evidence to discern the effect of PSLs on transfusion safety. Various bag and storage media changes have been proposed to reduce glycolysis and platelet activation during room temperature storage. Moreover, cryopreservation and cold storage have been proposed as potential methods to prolong PC shelf life by reducing platelet metabolism and bacterial proliferation. However, further work is required to elucidate and manage the PSLs specific to these storage protocols before its implementation in blood banks.

A proteomic approach reveals the variation in human platelet protein composition after storage at different temperatures

Cryopreservation can slow down the metabolism and decrease the risk of bacterial contamination. But, chilled platelets (PLTs) show a reduced period in circulation due to the rapid clearance by hepatic cells or spleen macrophages after transfusion. The deleterious changes that PLTs undergo are mainly considered the result of PLT protein variation. However, the basis for proteomic variation of stored PLTs remains poorly understood. Besides count, activation markers (CD62P and Annexin V), and aggregation, we used quantitative mass spectrometry to create the first comprehensive and quantitative human PLT proteome of samples stored at different temperatures (22°C, 10°C and -80°C). We found different conditions caused different platelet storage lesion (PSL). PLT count was decreased no matter at what temperature stored. PLTs viability at low temperature dropped by 21.78% and 11.21%, respectively, as compared 10.26% at room temperature, there were no significant differences between the storage methods. Membrane expression of CD62P gradually increased in all groups especially stored at 22°C up to 40% and 10°C up to 30%. However, exposure of PS on the PLT membrane was below 1% in every group. The PLT proteome showed there were 575 and 454 potential proteins identified by general iTRAQ analysis and phosphorylation iTRAQ a nalysis, respectively, among them, 33 common differentially expressed proteins caused by storage time and 44 caused by storage temperature Especially, membrane-bound proteins (such as FERMT3, STX4, MYL9 and TAGLN2) played key roles in PLT storage lesion. The pathways "Endocytosis", "Fc gamma R-mediated phagocytosis" and "Regulation of actin cytoskeleton" were affected predominantly by storage time. And the pathways "SNARE interactions in vesicular transport" and "Vasopressin-regulated water reabsorption" were affected by cold storage in our study. Proteomic results can help us to understand PLT biochemistry and physiology and thus unravel the mechanisms of PSL in time and space for more successful PLT transfusion therapy.

Regulation of platelet lifespan by apoptosis

The lifespan of platelets in circulation is brief, close to 10 days in humans and 5 days in mice. Bone marrow residing megakaryocytes produce around 100 billion platelets per day. In a healthy individual, the majority of platelets are not consumed by hemostatic processes, but rather their lifespan is controlled by programmed cell death, a canonical intrinsic apoptosis program. In the last decade, insights from genetically manipulated mouse models and pharmacological developments have helped to define the components of the intrinsic, or mitochondrial, apoptosis pathway that controls platelet lifespan. This review focuses on the molecular regulation of apoptosis in platelet survival, reviews thrombocytopenic conditions linked to enhanced platelet death, examines implications of chemotherapy-induced thrombocytopenia through apoptosis-inducing drugs in cancer therapy as well as discusses ex vivo aging of platelets.

The role of apoptosis in megakaryocytes and platelets

The role of apoptotic pathways in the development and function of the megakaryocyte lineage has generated renewed interest in recent years. This has been driven by the advent of BH3 mimetic drugs that target BCL2 family proteins to induce apoptosis in tumour cells: agents such as ABT-263 (navitoclax, which targets BCL2, BCL-XL [BCL2L1] and BCL2L2) and ABT-199 (a BCL2-specific agent) are showing great promise in early stage clinical trials. However, the major dose-limiting toxicity of navitoclax has proven to be thrombocytopenia, an on-target effect of inhibiting BCL-XL . It transpires that the anucleate platelet contains a classical intrinsic apoptosis pathway, which at steady state regulates its life span in the circulation. BCL-XL is the critical pro-survival protein that restrains apoptosis and maintains platelet viability. These findings have paved the way to a deeper understanding of apoptotic pathways and processes in platelets, and their precursor cell, the megakaryocyte.

Therapeutic drug-induced platelet apoptosis: an overlooked issue in pharmacotoxicology

The surfacing of the applied fields of biology such as, biotechnology, pharmacology and drug discovery was a boon to the modern man. However, it had its share of disadvantages too. The indiscriminate use of antibiotics and other biological drugs resulted in numerous adverse reactions including thrombocytopenia. One of the reasons for drug-induced thrombocytopenia could be attributed to an enhanced rate of platelet apoptosis, which is a less investigated aspect. The present essay sheds light on the adverse (pro-apoptotic) effects of some of the commonly used drugs and antibiotics on platelets viz. cisplatin, aspirin, vancomycin and balhimycin. Furthermore, the undesirable reactions resulting from chemotherapy could be attributed at least to some extent to the systemic stress induced by microparticles, which in turn are the byproducts of platelet apoptosis. Thereby, the essay aims to highlight the challenges in the emerging trend of cross-disciplinary implications, i.e., drug-induced platelet apoptosis, which is a nascent field. Thus, the different mechanisms through which drugs induce platelet apoptosis are discussed, which also opens up a new perspective through which the adverse effects of commonly used drugs could be dealt. The drug-associated platelet toxicity is of grave concern and demands immediate attention. Besides, it would also be appealing to examine the platelet pro-apoptotic effects of other commonly used therapeutic drugs.

Omi/HtrA2 and XIAP are components of platelet apoptosis signalling

Although platelets possess the hallmarks of apoptosis such as activation of caspases, cytochrome c release and depolarisation of the mitochondrial transmembrane potential (∆Ψm), their entire apoptotic-signalling pathway is not totally understood. Therefore we studied the expression of various apoptotic proteins and found that platelets contain the pro-apoptotic proteins Omi/HtrA2 and Smac/Diablo, as well as their target the X-linked inhibitor of apoptosis XIAP. Omi/HtrA2 and Smac/Diablo were released from mitochondria into the platelet cytosol together with cytochrome c after induction of apoptosis by the Ca2+ ionophore A23187 or the BH3 mimetic ABT-737, and to a lesser extent, after platelet stimulation with collagen and thrombin. Inhibition of Omi/HtrA2 led to decreased levels of activated caspase-3/7 and caspase-9, but did not abolish loss of ∆Ψm or prevent release of Omi/HtrA2 from mitochondria. These results indicate that platelets have a functional intrinsic apoptotic-signalling pathway including the pro-apoptotic protease Omi/HtrA2 and its target protein XIAP.

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.

Alzheimer disease and platelets: how's that relevant

Alzheimer Disease (AD) is the most common neurodegenerative disorder worldwide, and account for 60% to 70% of all cases of progressive cognitive impairment in elderly patients. At the microscopic level distinctive features of AD are neurons and synapses degeneration, together with extensive amounts of senile plaques and neurofibrillars tangles. The degenerative process probably starts 20-30 years before the clinical onset of the disease. Senile plaques are composed of a central core of amyloid β peptide, Aβ, derived from the metabolism of the larger amyloid precursor protein, APP, which is expressed not only in the brain, but even in non neuronal tissues. More than 30 years ago, some studies reported that human platelets express APP and all the enzymatic activities necessary to process this protein through the same pathways described in the brain. Since then a large number of evidence has been accumulated to suggest that platelets may be a good peripheral model to study the metabolism of APP, and the pathophysiology of the onset of AD. In this review, we will summarize the current knowledge on the involvement of platelets in Alzheimer Disease. Although platelets are generally accepted as a suitable model for AD, the current scientific interest on this model is very high, because many concepts still remain debated and controversial. At the same time, however, these still unsolved divergences mirror a difficulty to establish constant parameters to better defined the role of platelets in AD.

Pathogen reduction treatment of buffy coat platelet concentrates in additive solution induces proapoptotic signaling

BACKGROUND

Pathogen reduction technology (PRT) can potentially reduce the risk of transfusion-transmitted infections. However, PRT treatment of platelet (PLT) concentrates also results in reduced PLT quality and increased markers of apoptosis during storage. The aim of this study was to investigate changes to the expression and activation of proteins involved in apoptosis signaling.

STUDY DESIGN AND METHODS

Samples from riboflavin and ultraviolet light PRT-treated and untreated (control) buffy coat-derived PCs in 70% SSP+ and 30% plasma were taken on Days 1, 5, and 7 of storage. Phosphatidylserine (PS) exposure, expression of Bcl-2 family proteins, cytochrome c release, and cleavage of caspase-3 and caspase-3 substrates were analyzed using flow cytometry and Western blotting.

RESULTS

Compared to untreated controls, markers of apoptosis signaling were increased after PRT and subsequent storage. PS exposure on the PLT outer membrane was significantly higher after PRT on Days 5 and 7 of storage (p < 0.05). Expression of proapoptotic Bak and Bax was higher after PRT and subsequent storage. Cytochrome c release and caspase-3 cleavage were also greater and occurred earlier in the PRT-treated PLTs. The cleavage of caspase-3 substrates gelsolin and ROCK I were also increased after PRT, compared to untreated controls.

CONCLUSIONS

This study demonstrated an increase in proapoptotic signaling during PLT storage, which was exacerbated by PRT. Many of these differences emerged outside the current 5-day storage period. These changes may not currently influence PLT transfusion quality, but will need to be carefully evaluated when considering extending PLT storage beyond 5 days.

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.

Apoptotic processes in megakaryocytes and platelets

It is becoming increasingly clear that most mammalian cells are capable of undergoing apoptosis and that, within particular lineages, specific apoptotic pathways have evolved to regulate survival and turnover. The role of apoptosis in the megakaryocyte lineage is an intriguing one. Various insults, such as chemotherapeutics, autoantibodies, and human immunodeficiency virus (HIV), have been suggested to induce the apoptotic death of megakaryocytes and/or their progenitors. Conversely, apoptotic processes have been implicated in megakaryocyte development and platelet production. Platelets also contain functional apoptotic pathways, which circumscribe their survival. It has even been suggested that platelet activation responses involve components of the apoptotic machinery, highlighting a potential role for apoptotic processes in hemostasis and thrombosis. This review discusses the current state of knowledge about how apoptosis and apoptotic proteins contribute to the generation and function of megakaryocytes and platelets.

Activation of PAK1/2 during the shedding of platelet microvesicles

Simultaneously to phospholipid flip-flop that supports the procoagulant activity of activated platelets, blebs, supported by actin reorganization, are formed at the plasma membrane and generate microvesicles. The molecular mechanism of microvesicle shedding from activated platelets implicates Ca influx and Ca-dependent protease, calpain. We previously demonstrated that the formation of lamellipodias and filopodias associated with platelet shape change involved the reorganization of actin filaments through a Cdc42/Rac1/p21-activated kinase (PAK)-dependent pathway. Here, we investigated whether platelet blebbing also depends on the Cdc42/Rac1/PAK pathway. Exposure of platelets in vitro to either a mixture of thrombin receptor-activating peptide (TRAP) and collagen or the Ca ionophore A23187 in the presence of Ca generates microvesicles that can be identified by flow cytometry. The calpain inhibitor, calpeptin, diminished microvesicle formation induced by the Ca ionophore A23187, confirming the role of calpain in this process. PAK1/2 is cleaved in a calpain-dependent manner, and calpeptin prevents this cleavage and allows a transient activation of the kinase. Inhibition of Cdc42 and Rac1 by toxin B from Clostridium difficile, that suppresses PAK1/2 activation induced by TRAP and collagen or by A23187 in the presence of calpeptin, decreases polymerization of actin, lamellipodia and filopodia formation and interferes with the shedding of microvesicles. We conclude that the Rac1/Cdc42/PAK pathway controls actin reorganization that is necessary for microvesicle shedding.

Procoagulant platelets: are they necrotic?

Apoptosis and necrosis represent distinct cell death processes that regulate mammalian development, physiology and disease. Apoptosis characteristically leads to the silent destruction and removal of cells in the absence of an inflammatory response. In contrast, necrotic cell death can induce physiologic inflammatory responses linked to tissue defense and repair. Although anucleate, platelets undergo programmed cell death, with apoptosis playing an important role in clearing effete platelets from the circulation. While it has long been recognized that procoagulant platelets exhibit characteristic features of dying cells, recent studies have demonstrated that platelet procoagulant function can occur independent of apoptosis. A growing body of evidence suggest that the biochemical, morphologic and functional changes underlying agonist-induced platelet procoagulant function are broadly consistent with cell necrosis, raising the possibility that distinct death pathways regulate platelet function and survival. In this article, we will discuss the mechanisms underlying apoptotic and necrotic cell death pathways and examine the evidence linking these pathways to the platelet procoagulant response. We will also discuss the potential contribution of these pathways to the platelet storage lesion and propose a simplified nomenclature to describe procoagulant platelets.

Storage of platelet concentrates from pooled buffy coats made of fresh and overnight-stored whole blood processed on the novel Atreus 2C+ system: in vitro study

BACKGROUND

The Atreus 2C+ system (Gambro BCT) automatically separates whole blood (WB) into buffy coat (BC), red blood cells (RBC), and plasma and transfers the components into separate containers. After processing with the Atreus, 4 to 6 BC units can be pooled and processed into leukoreduced platelets (PLTs) by use of the automated OrbiSac BC system (Gambro BCT). The aim of our in vitro study was to investigate the effects of holding either WB or BC overnight before preparation of PLTs by use of the Atreus 2C+ system for BC preparation. A standard routine procedure involving conventional blood containers for the preparation of BC combined with the OrbiSac process (top-and-top system; Terumo) was used as a reference.

STUDY DESIGN AND METHODS

WB was either processed within 8 hours after collection ("fresh blood") or stored overnight before processing. WB units were separated into BC, RBC, and plasma units and transferred into individual containers. Either the BC or the WB units rested overnight at 22 +/- 2 degrees C. Six ABO-identical BCs, obtained from either fresh or overnight-stored WB, were pooled and processed with the OrbiSac BC system to obtain leukoreduced PLTs. In total, 20 Atreus and 10 reference (leukoreduced PLTs) samples were analyzed for various in vitro variables during the 7-day storage period.

RESULTS

No significant difference in glucose consumption, lactate production, mean PLT volume, LDH activity, bicarbonate, ATP, RANTES, and the expression of CD62p and CD42b between groups was detected. pH was maintained at greater than 7.0 (Day 7). Swirling remained at the highest levels (score, 2) for all units throughout storage.

CONCLUSION

PLTs derived from BCs, obtained from either fresh or overnight-stored WB processed on the novel automated Atreus 2C+ system, were equivalent to control PLTs with regard to PLT in vitro characteristics during 7 days of storage. Stable recovery of PLTs and satisfactory PLT content according to current standards were also found.

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.

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.

Proteolysis leads to the appearance of the long form of beta3-endonexin in human platelets

After vessel injury, platelets adhere to the subendothelial matrix. Platelet adhesion leads to activation of the platelet integrin alpha(IIb)beta3, which then binds to fibrinogen, leading to platelet aggregation. It has been shown that a beta3-integrin binding protein, beta3-endonexin, can activate the integrin alpha(IIb)beta3 expressed in transfected CHO cells. Several isoforms of beta3-endonexin are known but it is not clear which isoforms are expressed in platelets and what role they may play during haemostasis. Here, we show that the long form of beta3-endonexin (EN-L) can be detected in platelet lysates several hours after thrombus formation, after long-term storage of platelets and after glucose deprivation. After subcellular fractionation, EN-L is found in the detergent insoluble fraction suggesting that it might be associated with the cytoskeleton. EN-L generation is temperature and Ca++ dependent and requires physiological salt concentrations. Proteolysis is responsible for the appearance of EN-L since a calpain inhibitor prevents its formation and the addition of calpain to platelet lysates induces its formation. The appearance of EN-L seems to be linked to apoptotic events occurring during long-term storage of platelets and, possibly, during late steps of haemostasis after thrombus formation.

BimEL up‐regulation potentiates AIF translocation and cell death in response to MPTP

This study attempted to elucidate the signaling mechanism underlying dopaminergic cell death in the MPP+ model for Parkinson's disease. In neuronal-differentiated PC12 cells, through the regulation by activated JNK and c-jun, BimEL expression was markedly increased in response to MPP+ treatment, which led to the cell degeneration. In lieu of Smac translocation as seen in other paradigms, up-regulation of BimEL effected an increase in calpain I activity that, in turn, mediated AIF release from the mitochondria. In support, we found that knocking down BimEL expression resulted in a decrease in calpain I activity, as well as AIF release from the mitochondria and cell death. Finally, inhibition of calpain activity mitigated AIF release from the mitochondria and cell death. Under cell-free conditions, activated purified calpain I could induce the release of AIF from isolated mitochondria without the participation of BimEL or activated JNK, suggesting that AIF release is a direct consequence of calpain I activity. In concert, the results suggest a novel signaling pathway for dopaminergic cell degeneration, in which MPP+ induces the up-regulation of BimEL, which in turn potentiates an elevation in calpain I activity that mediates AIF release and cell death in a caspase-independent manner.

Life's smile, death's grin: vital functions of apoptosis-executing proteins

Apoptosis is executed by caspases as well as caspase-independent death effectors. Caspases are expressed as inactive zymogens in virtually all animal cells and are activated in cells destined to undergo apoptosis. However, there are many examples where caspase activation is actually required for cellular processes not related to cell death, namely terminal differentiation, activation, proliferation, and cytoprotection. Several caspase-independent death effectors including apoptosis-inducing factor, endonuclease G and a serine protease (Omi/HtrA2) are released from the mitochondrial intermembrane space upon permeabilization of the outer membrane. Such proteins also have important roles in cellular redox metabolism and/or mitochondrial biogenesis. As a general rule, it thus appears that cell-death-relevant proteins, especially those involved in the core of the executing machinery, have a dual function in life and death. This has important implications for pathophysiology. The fact that the building blocks of the apoptotic machinery have normal functions not related to cell death may mean that essential parts of the apoptotic executioner cannot be lost and thus reduces the possibility of oncogenic mutations that block the apoptotic program. Moreover, therapeutic suppression of unwarranted cell death must be designed to target only the lethal (and not the vital) role of death effectors.

Pathologic high shear stress induces apoptosis events in human platelets

Recently, it has been discovered that apoptosis of anucleate platelets can be induced by chemical agonists. Other studies demonstrated that mechanical forces (shear stresses) stimulate platelet activation and signaling in the absence of exogenous chemical stimuli. We analyzed whether shear stresses can trigger platelet apoptosis, a question that has not yet been studied. Using a cone-and-plate viscometer, we exposed human platelet-rich plasma to different shear stresses, ranging from physiologic arterial and arteriole levels (10-44 dyn/cm2) to pathologic high levels (117-388 dyn/cm2) occurring in stenotic vessels. We found that pathologic shear stresses induce not only platelet activation (P-selectin upregulation and GPIbalpha downregulation) but also trigger apoptosis events, including mitochondrial transmembrane potential depolarization, caspase 3 activation, phosphatidylserine exposure, and platelet shrinkage and fragmentation, whereas physiological shear stresses are not effective. This novel finding suggests that shear-induced platelet apoptosis can be mediated by mechanoreceptors, does not require nuclear participation, and may affect platelet clearance.

Progressive resistance of BTK-143 osteosarcoma cells to Apo2L/TRAIL-induced apoptosis is mediated by acquisition of DcR2/TRAIL-R4 expression: resensitisation with chemotherapy

Apo2 ligand (Apo2L, also known as TRAIL) is a member of the tumour necrosis factor (TNF) family of cytokines that selectively induces the death of cancer cells, but not of normal cells. We observed that recombinant Apo2L/TRAIL was proapoptotic in early-passage BTK-143 osteogenic sarcoma cells, inducing 80% cell death during a 24 h treatment period. Apo2L/TRAIL-induced apoptosis was blocked by caspase inhibition. With increasing passage in culture, BTK-143 cells became progressively resistant to the apoptotic effects of Apo2L/TRAIL. RNA and flow cytometric analysis demonstrated that resistance to Apo2L/TRAIL was paralleled by progressive acquisition of the decoy receptor, DcR2. Blocking of DcR2 function with a specific anti-DcR2 antibody restored sensitivity to Apo2L/TRAIL in a dose-dependent manner. Importantly, treatment of resistant cells with the chemotherapeutic agents doxorubicin, cisplatin and etoposide reversed the resistance to Apo2L/TRAIL, which was associated with drug-induced upregulation of mRNA encoding the death receptors DR4 and DR5. BTK-143 cells thus represent a useful model system to investigate both the mechanisms of acquisition of resistance of tumour cells to Apo2L/TRAIL and the use of conventional drugs and novel agents to overcome resistance to Apo2L/TRAIL.

Platelet apoptosis in stored platelet concentrates and other models

Apoptosis or programmed cell death was discovered in nucleate cells 30 years ago and has been well documented. In contrast, apoptosis in anucleate platelets has only a five-year research history and as yet but few publications related to it. In this review, we will present the data on platelet apoptosis in several models. These include in vitro models where platelet apoptosis was induced by calcium ionophores, natural platelet agonists, storage in capped tubes at 37 degrees C and storage at room temperature under standard blood banking conditions, and in vivo models where apoptosis was provoked by suppression of thrombopoiesis, malaria infection and injection of tumor necrosis factor or anti-platelet antibodies. Understanding of platelet apoptosis and its role in the platelet storage lesion is an exciting challenge; future research is likely to provide us with further insight into this field.

Apoptotic activity in stored human platelets

BACKGROUND

Platelets possess some of the machinery required for apoptotic cell death. However, disruption of mitochondria function, implicated in several models of cell death, has not been extensively studied in platelets. Mitochondrial viability and several other measures of apoptotic death in stored and experimentally stressed platelets were evaluated.

MATERIALS AND METHODS

Platelet mitochondrial transmembrane potentials (Deltapsim) were studied by staining platelets with JC-1, a dye that fluoresces at different wavelengths based on the state of mitochondrial polarization. Annexin V binding, a measure of phosphatidylserine (PS) exposure, and CD62P expression, an indicator of platelet activation, were determined by flow cytometry. Caspase-3 activity was measured with an enzyme assay and by Western blotting. Experimental platelet stressors included storage for 7 days, azide exposure, calcium ionophore stimulation, and plasma deprivation.

RESULTS

As measured by flow cytometry, Deltapsim values were similar in freshly drawn platelets and in platelet concentrates stored for up to 7 days. However, compared to fresh platelets, stored platelet concentrates had significantly increased PS exposure (3.1 vs. 5.1%, p = 0.015), CD62P expression (6.5 vs. 13.5%, p = 0.0067), and caspase-3 activity. Azide exposure, which decreased ATP release 20 to 30 percent, did not affect the Deltapsim. Stressed platelets exhibited higher degrees of mitochondrial depolarization in response to calcium ionophore stimulation than platelets that were not stressed. Plasma deprivation also resulted in significant alterations in Deltapsim, PS exposure, and CD62P expression.

CONCLUSIONS

Platelet mitochondria maintain Deltapsim when stored for up to 7 days under standard blood bank storage conditions. Therefore, changes in platelet mitochondria Deltapsim do not correlate with downstream markers of apoptotic death such as caspase activation and PS exposure.

Tributyltin (TBT) induces ultra-rapid caspase activation independent of apoptosome formation in human platelets

Activation of caspases has been demonstrated to be involved in thrombocytopenia and prolonged storage of platelet concentrates. Platelets represent enucleate cells that comprise all elements of the mitochondrial apoptosis pathway. However, no apoptotic stimuli capable of activating the endogenous caspase cascade have been identified so far. Using tributyltin (TBT) we could identify a compound that is capable of activating caspase-9 and -3 in platelets. Recent studies implicate that TBT induces apoptosis via the mitochondrial signaling pathway that is characterized by the formation of a high-molecular-weight complex (apoptosome) containing the adapter protein Apaf-1 and active caspase-9. Interestingly, addition of TBT induced the activation of caspase-9 in an ultra-rapid kinetic within the first 2 min. In addition, size exclusion chromatography revealed that TBT-mediated processing of caspase-9 occurs in the absence of the apoptosome. Thus, these data implicate that TBT induces the activation of caspase-9 by a mechanism not involving the formation of the apoptosome.