Apoptotic markers are increased in platelets stored at 37 degrees C

Platelet PD-L1 inhibits storage-induced apoptosis by sustaining activation of the AKT signalling pathway

Platelet apoptosis is irreversible under current storage conditions in blood banks. Studies have shown that programmed cell death ligand 1 (PD-L1) in tumour cells is required for neoplastic progression, tumour recurrence and metastasis by regulating apoptosis. However, whether PD-L1 is involved in storage-induced apoptosis in platelets remains poorly understood. In this study, we explored whether PD-L1 on platelets participated in the regulation of storage-induced apoptosis under blood bank conditions, as well as the underlying mechanism. Several apoptotic events in platelets from humans and PD-L1-knockout mice during storage under blood bank conditions were measured. The mechanism by which storage-induced apoptosis was regulated by platelet-intrinsic PD-L1 signalling was further investigated. Our results showed that PD-L1 in platelets progressively decreased. There was a strong negative correlation between platelet PD-L1 expression and the phosphatidylserine (PS) externalization rate and cleaved caspase-3 level and a positive correlation with anti-apoptosis protein Bcl-xl. Ex vivo, PD-L1-/- platelets stored at 22 °C showed rapid apoptosis via an intrinsic mitochondria-dependent pathway over time. Likewise, inhibiting PD-L1 signalling with BMS-1166 accelerated apoptosis by intrinsic mitochondria-dependent pathway. Coimmunoprecipitation analysis revealed that PD-L1 could bind AKT in platelets, and the binding capacity of both showed a progressive decrease with time. Finally, the decrease in PD-L1 expression levels during storage could be attributed to a complex process of progressive secretion. Therefore, platelet PD-L1 inhibits storage-induced apoptosis by sustaining activation of the AKT signalling pathway, which is expected to become a target for alleviating platelet storage lesions (PSLs) under current blood bank conditions.

L-carnitine contributes to enhancement of viability and quality of platelet concentrates through changing the apoptotic and anti-apoptotic associated microRNAs

BACKGROUND

Micro RNAs are known as the main regulator of messenger RNA translation in platelets and have a vital role in process of apoptosis during platelet storage. Our pervious study revealed that the expression of miR-145 and miR-326 changed significantly in platelets under maintenance conditions. This study aimed to evaluate the effect of L-carnitine (LC) as an additive to augment platelet quality by changing the microRNA expression.

METHODS

We used ten platelet concentrate (PC) bags and divided each into two equal parts, LC- treated, and LC free PC. The expression of miR-145 and miR-326 were determined using real-time PCR. Moreover, we measured platelet count, platelet aggregation, platelet viability, and lactate dehydrogenase activity in all samples.

RESULTS

The miR-326 expression significantly increased during platelet storage with mean fold changes of 3.2 for the control and 2.5 for LC- treated PC. The mean fold changes in miR-145 expression was less in the control PC (0.52) compared to the LC- treated PC (0.79). Increased levels of platelet count, platelet aggregation, and platelet viability were found in the LC-treated compared to the untreated PC.

CONCLUSION

LC has a protective effect on platelet apoptosis, reduces the expression of apoptotic microRNA, and prevents the reduction of anti-apoptotic microRNA.

Structural analysis of platelet fragments and extracellular vesicles produced by apheresis platelets during storage

ABSTRACT

Platelets (PLTs) for transfusion can be stored for up to 7 days at room temperature (RT). The quality of apheresis PLTs decreases over storage time, which affects PLT hemostatic functions. Here, we characterized the membranous particles produced by PLT storage lesion (PSLPs), including degranulated PLTs, PLT ghosts, membrane fragments, and extracellular membrane vesicles (PEVs). The PSLPs generated in apheresis platelet units were analyzed on days 1, 3, 5, and 7 of RT storage. A differential centrifugation and a sucrose density gradient were used to separate PSLP populations. PSLPs were characterized using scanning and transmission electron microscopy (EM), flow cytometry (FC), and nanoparticle tracking analysis (NTA). PSLPs have different morphologies and a broad size distribution; FC and NTA showed that the concentration of small and large PSLPs increases with storage time. The density gradient separated 3 PSLP populations: (1) degranulated PLTs, PLT ghosts, and large PLT fragments; (2) PEVs originated from PLT activation and organelles released by necrotic PLTs; and (3) PEV ghosts. Most PSLPs expressed phosphatidyl serine and induced thrombin generation in the plasma. PSLPs contained extracellular mitochondria and some had the autophagosome marker LC3. PSLPs encompass degranulated PLTs, PLT ghosts, large PLT fragments, large and dense PEVs, and low-density PEV ghosts. The activation-related PSLPs are released, particularly during early stage of storage (days 1-3), and the release of apoptosis- and necrosis-related PSLPs prevails after that. No elevation of LC3- and TOM20-positive PSLPs indicates that the increase of extracellular mitochondria during later-stage storage is not associated with PLT mitophagy.

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.

Gamma irradiation induces a pro-apoptotic state in longer stored platelets, without progressing to an overt apoptosis by day 7 of storage

BACKGROUND

Although gamma-irradiation to platelet products is a standard method to prevent the risk of TA-GVHD in vulnerable recipients, it induces some proteomic and redox changes, of which irradiation-induced ROS increments may potentiate platelet mitochondrial dysfunction. However, whether these changes cause platelet apoptosis, or affect their viability during storage, is the main subject of this study.

METHODS

PLT-rich plasma PC was split into two bags, one kept as control while other was subjected to gamma-irradiation. Within 7-days storage, cytosolic and mitochondrial levels of cytochrome c and pro-apoptotic molecules of Bak and Bax were evaluated by western-blotting. Intraplatelet active caspase (using FAM-DEVD-FMK) and PS-exposure were detected by flowcytometry. Caspase activity in platelet lysate was also confirmed by immunofluorescence detection of Caspase-3/7 Substrate N-Ac-DEVD-N'-MC-R110 while platelet viability was evaluated with MTT assays.

RESULTS

Cytosolic cytochrome c gradually increased while its mitochondrial content steadily declined during 7 days of storage. In a contrary trend, reverse patterns were observed for Bak and Bax expressions. Gamma-irradiated platelets showed higher release of mitochondrial cytochrome c that reflected by higher cytosolic cytochrome c levels on day 7 of storage. Concurrently mitochondrial pro-apoptotic Bak and Bax proteins increased on day 7 in irradiated products. However, gamma-irradiation didn't significantly increase caspase activity or PS-exposure, nor did it decrease platelet viability.

CONCLUSION

Here, consistent with studies on "gamma-irradiation-induced oxidative stress", we showed that gamma-ray also increases platelet pro-apoptotic signals during storage, although not strongly enough to affect platelet viability by overt apoptosis induction. Conclusively, whether supplementing ROS scavengers or antioxidants to irradiated platelets can improve their quality during storage may be of interest for future research.

Determining the Optimal Storage Time and Temperature for Performing Platelet Function Assays and Global Hemostasis Assays

Platelet function assays and global haemostasis assays are essential in diagnosing bleeding tendencies, with light transmission aggregometry (LTA) as golden standard. The Multiple Electrode Aggregation (Multiplate), platelet function assay (PFA) and rotational thromboelastometry (ROTEM) are mostly used as whole-blood screening tests. Currently, patients have to travel to specialized laboratories to undergo these tests, since specific expertise is required. Pre-analytical variables, like storage time and temperature during transport, are still considered to be the most vulnerable part of the process and may lead to discrepancies in the test results. We aim to give a first impression on the stability of blood samples from healthy volunteers during storage and investigate the effect of storage time (1, 3, 6 and 24 hours) and temperature (4°C, room temperature and 37°C) on the Multiplate, PFA, ROTEM and LTA test results. Our data indicated that, for the PFA, whole blood can be stored for 3 hours at room temperature. Whole blood used for the Multiplate and ROTEM can be stored for 6 hours of storage. For LTA, PRP and whole blood were stable up to 3 hours at 4°C or room temperature and 6 hours at room temperature, respectively.

Apoptotic profiling of chronic myeloid leukaemia patients' platelets ex vivo before and after treatment with Imatinib

Chronic myeloid leukaemia (CML) is a malignancy of the haematopoietic stem cells. The first line of treatment for CML, especially in developing countries, remains the first-generation tyrosine kinase inhibitor, Imatinib. Patients with CML are frequently diagnosed with platelet abnormalities. However, the specific mechanism of platelet abnormalities in CML remains unclear and poorly understood. The aim of this study was therefore to determine the apoptotic profiles of CML patients ex vivo on platelets before and after treatment with Imatinib. Blood samples of healthy volunteers and CML patients at diagnosis and after 6 months treatment with Imatinib were collected. Platelet counts, viability and activation were determined. Results showed that CML patients' platelet counts were elevated upon diagnosis and these levels statistically significantly decreased after 6 months of treatment. Platelet activation was significantly increased after 6 months of treatment compared to levels at diagnosis (P-value < .05). Similarly, platelet apoptosis was also increased after 6 months of treatment. Abnormalities in platelet functioning found in this study may partly be due to clonal proliferation of haematopoietic cells in CML patients, specifically of megakaryocyte precursors as well as the inhibition of platelet tyrosine kinase's and the inhibition of platelet-derived growth factor.

Regulation of Platelet Production and Life Span: Role of Bcl-xL and Potential Implications for Human Platelet Diseases

Blood platelets have important roles in haemostasis, where they quickly stop bleeding in response to vascular damage. They have also recognised functions in thrombosis, immunity, antimicrobal defense, cancer growth and metastasis, tumour angiogenesis, lymphangiogenesis, inflammatory diseases, wound healing, liver regeneration and neurodegeneration. Their brief life span in circulation is strictly controlled by intrinsic apoptosis, where the prosurvival Bcl-2 family protein, Bcl-xL, has a major role. Blood platelets are produced by large polyploid precursor cells, megakaryocytes, residing mainly in the bone marrow. Together with Mcl-1, Bcl-xL regulates megakaryocyte survival. This review describes megakaryocyte maturation and survival, platelet production, platelet life span and diseases of abnormal platelet number with a focus on the role of Bcl-xL during these processes.

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.

Platelets Apoptosis and Clearance in The Presence of Sodium Octanoate during Storage of Platelet Concentrate at 4˚C

Objective

Platelet (PLT) storage at 4˚C has several benefits, however, it is accompanied by increased clearance of PLTs after transfusion. In this study, we evaluated the potential of sodium octanoate (SO) for reducing apoptosis and clearance rate of PLTs after long-term storage in cold.

Materials and Methods

In this experimental study, PLT concentrates (PCs) were stored for 5 days under the following three conditions: 20-24˚C, 4˚C, and 4˚C in the presence of SO. To measure the viability of PLTs, the water-soluble tetrazolium salt (WST-1) assay was performed. Phosphatidylserine (PS) exposure was determined on PLTs using flow cytometry technique. The amount of human active caspase-3 was determined in PLTs using an enzyme-linked immunosorbent assay. Additionally, the amount of PLT ingestion or clearance was determined by using HepG2 cell line.

Results

The viability was higher in the SO-treated PLTs compared to the other groups. The level of PS exposure on PLTs was lower in the SO-treated PLTs compared to the other groups. The amount of active caspase-3 increased in all groups during 5-day storage. The highest increase in the amount of caspase-3 levels was observed at cold temperature. However, PLTs kept at 4˚C in the presence of SO had a lower amount of active caspase-3 compared to PLTs kept at 4˚C. The amount of PLTs removal by HepG2 cells was increased for 4˚C-kept PLTs but it was lower for PLTs kept at 4˚C in the presence of SO but, the differences were not significant (P>0.05).

Conclusion

SO could partially moderate the effects of cold temperature on apoptosis and viability of platelets. It also decreases the ingestion rate of long-time refrigerated PLTs in vitro. Further studies using higher numbers of samples are required to demonstrate the effect of SO on reducing the clearance rate of PLTs.

Platelet factor 4-containing immune complexes induce platelet activation followed by calpain-dependent platelet death

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy sometimes associated with thrombosis. The hallmark of HIT is antibodies to the heparin/platelet factor 4 (PF4) complex that cause thrombocytopenia and thrombosis through platelet activation. Despite the clinical importance, the molecular mechanisms and late consequences of immune platelet activation are not fully understood. Here, we studied immediate and delayed effects of the complexes formed by human PF4 and HIT-like monoclonal mouse anti-human-PF4/heparin IgG antibodies (named KKO) on isolated human platelets in vitro. Direct platelet-activating effect of the KKO/PF4 complexes was corroborated by the overexpression of phosphatidylserine (PS) and P-selectin on the platelet surface. The immune platelet activation was accompanied by a decrease of the mitochondrial transmembrane potential (ΔΨm), concurrent with a significant gradual reduction of the ATP content in platelets, indicating disruption of energy metabolism. A combination of PS expression and mitochondrial depolarization induced by the PF4-containing immune complexes observed in a substantial fraction of platelets was considered as a sign of ongoing platelet death, as opposed to a subpopulation of activated live platelets with PS on the plasma membrane but normal ΔΨm. Both activated and dying platelets treated with KKO/PF4 formed procoagulant extracellular microvesicles bearing PS on their surface. Scanning and transmission electron microscopy revealed dramatic morphological changes of KKO/PF4-treated platelets, including their fragmentation, another indicator of cell death. Most of the effects of KKO/PF4 were prevented by an anti-FcγRII monoclonal antibody IV.3. The adverse functional and structural changes in platelets induced by the KKO/PF4 complexes were associated with strong time-dependent activation of calpain, but only trace cleavage of caspase 3. The results indicate that the pathogenic PF4-containing HIT-like immune complexes induce direct prothrombotic platelet activation via FcγRIIA receptors followed by non-apoptotic calpain-dependent death of platelets, which can be an important mechanism of thrombocytopenia during HIT development.

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.

Cyanidin-3-O-β-glucoside, a Natural Polyphenol, Exerts Proapoptotic Effects on Activated Platelets and Enhances Megakaryocytic Proplatelet Formation

This study investigated whether the anthocyanin cyanidin-3-O-β-glucoside (Cy-3-g) could affect platelet apoptosis and proplatelet formation in vitro. Thrombin-stimulated or resting human platelets and Meg-01 megakaryocytes were incubated with Cy-3-g (0, 0.5, 5, or 50 μM). We found that the percentage of the platelet mitochondrial membrane potential treated with 5 and 50 μM Cy-3-g was significantly higher than control (15.50% ± 3.24% and 29.77% ± 4.06% versus 2.76% ± 1.33%, respectively; P < 0.05). Treatment with 5 and 50 μM Cy-3-g significantly increased phosphatidylserine exposure compared with control (40.56% ± 10.53% and 76.62% ± 8.28% versus 15.43% ± 3.93%, respectively; P < 0.05). Moreover, Cy-3-g significantly increased the expression of Bax, Bak, and cytochrome c while markedly decreasing Bcl-xL and Bcl-2 expression as well as stimulating caspase-3, caspase-9, caspase-8, Bid, and gelsolin cleavage in thrombin-activated platelets in a dose-dependent manner ( P < 0.05). However, no significant differences were observed in the apoptosis of resting platelets when treated with Cy-3-g ( P > 0.05). Furthermore, Cy-3-g significantly ( P < 0.05) enhanced cell viability (50 μM versus control, 1.34 ± 0.01 versus 0.35 ± 0.02), the number of colony-forming unit-megakaryocytes (50 μM versus control, 38 ± 3 versus 8 ± 3), CD41 expression (50 μM versus control, 96.80% ± 2.55% versus 25.57% ± 2.86%), DNA ploidy (16N) (50 μM versus control, 19.73% ± 2.34% versus 4.42% ± 1.96%), and proplatelet formation (50 μM versus control, 27.5% ± 3.77% versus 7.67% ± 2.25%) in Meg-01 cells. In conclusion, Cy-3-g promotes activated platelet apoptosis and enhances megakaryocyte proliferation, differentiation, and proplatelet formation in vitro.

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.

Mitochondrial PIP3-binding protein FUNDC2 supports platelet survival via AKT signaling pathway

Platelets undergo apoptosis in response to a variety of stimuli in the circulation. Mitochondria in platelets are essential for their apoptosis. Specifically, pro-survival protein BCL-x_L on mitochondria is the key regulator of platelet lifespan. Here we identify an outer mitochondrial membrane protein FUNDC2 for platelet survival. FUNDC2 knockout mice carrying excessively apoptotic platelets exhibit thrombocytopenia in response to hypoxia. Mechanistically, FUNDC2 binds the lipid PIP3 via its unique, highly conserved N-terminal motif. FUNDC2 deficiency abrogates the phosphorylation of AKT and its substrate BAD in a PIP3/PI3K-dependent manner, which suppresses BCL-x_L. Indeed, FUNDC2 deficiency shortens the platelet lifespan under stress. Thus, this FUNDC2/AKT/BCL-x_L axis signifies a balance between platelet survival and apoptosis at the single organelle level and provides new insight for platelet-related diseases as well.

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.

βγ-CAT, a non-lens betagamma-crystallin and trefoil factor complex, induces calcium-dependent platelet apoptosis

In recent years, it has been reported that apoptosis may occur in platelets and play a role in the clearance of effete platelets. βγ-CAT, a newly identified non-lens βγ-crystallin and trefoil factor complex from frog Bombina maxima skin secretions, caused several in vivo toxic effects on mammals. Through determined haematological parameters of rabbits, it has been found that βγ-CAT significantly reduced the number of platelets in a time-dependent manner. Here, in order to explore the effect of βγ-CAT on platelets, washed platelets were incubated with various concentrations of βγ-CAT for 30 minutes. We found that βγ-CAT induced several apoptosis events in human platelets, including caspase-3 activation, phosphatidylserine (PS) exposure, depolarisation of mitochondrial inner transmembrane potential (ΔΨm), cytochrome c re-lease and strong expression of pro-apoptotic Bax and Bak proteins. However, βγ-CAT did not significantly induce platelet activation as detected by P-selectin surface expression, GPIIb/IIIa activation and platelet aggregation. In addition, we observed that βγ-CAT-induced PS exposure and ΔΨm depolarisation in platelets are Ca2+-dependent. Taken together, βγ-CAT can induce Ca2+-dependent platelet apoptosis but does not cause platelet activation.

An overview of the role of platelets in angiogenesis, apoptosis and autophagy in chronic myeloid leukaemia

Amongst males, leukaemia is the most common cause of cancer-related death in individuals younger than 40 years of age whereas in female children and adolescents, leukaemia is the most common cause of cancer-related death. Chronic myeloid leukaemia (CML) is a chronic leukaemia of the haematopoietic stem cells affecting mostly adults. The disease results from a translocation of the Philadelphia chromosome in stem cells of the bone marrow. CML patients usually present with mild to moderate anaemia and with decreased, normal, or increased platelet counts. CML represents 0.5% of all new cancer cases in the United States (2016). In 2016, an estimated 1070 people would die of this disease in the United States. Platelets serve as a means for tumours to increase growth and to provide physical- and mechanical support to elude the immune system and to metastasize. Currently there is no literature available on the role that platelets play in CML progression, despite literature reporting the fact that platelet count and size are affected. Resistance to CML treatment with tyrosine kinase inhibitors can be as a result of acquired resistance ensuing from mutations in the tyrosine kinase domains, loss of response or poor tolerance. In CML this resistance has recently become linked to bone marrow (BM) angiogenesis which aids in the growth and survival of leukaemia cells. The discovery of the lungs as a site of haematopoietic progenitors, suggests that CML resistance is not localized to the bone marrow and that the mutations leading to the disease and resistance to treatment may also occur in the haematopoietic progenitors in the lungs. In conclusion, platelets are significantly affected during CML progression and treatment. Investigation into the role that platelets play in CML progression is vital including how treatment affects the cell death mechanisms of platelets.

Mechanistic insight of platelet apoptosis leading to non-surgical bleeding among heart failure patients supported by continuous-flow left ventricular assist devices

Non-surgical bleeding (NSB) is the most common clinical complication in heart failure (HF) patients supported by continuous-flow left ventricular assist devices (CF-LVADs). In this study, oxidative stress and alteration of signal pathways leading to platelet apoptosis were investigated. Thirty-one HF patients supported by CF-LVADs were divided into bleeder (n = 12) and non-bleeder (n = 19) groups. Multiple blood samples were collected at pre-implant (baseline) and weekly up to 1-month post-implant. A single blood sample was collected from healthy subjects (reference). Production of reactive oxygen species (ROS) in platelets, total antioxidant capacity (TAC), oxidized low-density lipoproteins (oxLDL), expression of Bcl-2 and Bcl-xL, Bax and release of cytochrome c (Cyt.c), platelet mitochondrial membrane potential (Δψ _m), activation of caspases, gelsolin cleavage and platelet apoptosis were examined. Significantly elevated ROS, oxLDL and depleted TAC were evident in the bleeder group compared to non-bleeder group (p < 0.05). Platelet pro-survival proteins (Bcl-2, Bcl-xL) were significantly reduced in the bleeder group in comparison to the non-bleeder group (p < 0.05). Translocation of Bax into platelet mitochondria membrane and subsequent release of Cyt.c were more prevalent in the bleeder group. Platelet mitochondrial damage, activation of caspases, gelsolin cleavage, and ultimate platelet apoptosis in the bleeder group were observed. Oxidative stress and activation of both intrinsic and extrinsic pathways of platelet apoptosis may be linked to NSB in CF-LVAD patients. Additionally, biomarkers of oxidative stress, examination of pro-survivals and pro-apoptotic proteins in platelets, mitochondrial damage, caspase activation, and platelet apoptosis may be used to help identify HF patients at high risk of NSB post-implant.

Dengue Virus Entry and Replication Does Not Lead to Productive Infection in Platelets

Thrombocytopenia is a characteristic feature during the acute phase of dengue infection and has been found to associate with vascular leakage in severe dengue. Although dengue antigens have been observed in platelets, there is no strong evidence to suggest a direct infection of platelets by dengue virus as a contributing factor for thrombocytopenia. We show that dengue virus can enter platelets but replicate viral ribonucleic acid to a minimal extent and, therefore, cannot produce infectious virus. Dengue antigen was undetectable in platelets isolated from dengue patients; however, we observed an increase in CD14⁺CD16⁺ monocyte-platelet complexes, suggesting a mechanism for platelet clearance.

Blood derived products in pediatrics: New laboratory tools for optimizing potency assignment and reducing side effects

Neonates and children can develop rare bleeding disorders due to congenital/acquired coagulation Factor deficiencies, or allo-immune/autoimmune complications, or can undergo surgeries at high haemorrhagic risk. They then need specialized transfusion of blood components/products, or purified blood extracted products or recombinant proteins. Blood-derived therapies conventionally used for management of affected infants with genetic/acquired deficiencies, bleeding problems (coagulation Factor reduced or missing) or thrombotic disorders (reduced or missing anticoagulant proteins) pose some additional risks. These remedial therapies can cause tolerance when used very early in life and, sometimes needed, repeatedly. The introduction of recombinant proteins has allowed manufacturers to produce large amounts of the proteins usually present at very low concentration in blood. This has also changed the risk pattern of plasma-extracted products, especially in terms of continual reduction of viral transmission. Many efforts have been made over these past decades to reduce the risks associated with the use of all these products in terms of viral and bacterial safety, as well as immune disorders but they are not the objective of this article. Other associated side effects are the presence of undesired activities in blood products, which can produce thrombotic events or adverse reactions. The progressive introduction of blood derived products has greatly improved the prognosis and quality of life of affected patients. This concerns whole blood, but also blood cell concentrates, mainly platelets and red blood cells, plasma, while the blood extracted products are increasingly replaced by recombinant proteins. All these therapeutic products, i.e. blood extracted drugs, improve health and quality of life for hemophiliac's A or B, or patients with auto/allo-immune thrombocytopenias or with rare bleeding disorders, and those with thrombotic events occurring in childhood, which are mainly due to Protein C or Protein S deficiencies (congenital or acquired). Progress in analytical methods and biotechnology allow better control of the manufacturing processes for all blood derived or plasma extracted products and recombinant proteins, and contribute to improved manufacturing processes to minimize the occurrence of side effects. These adverse events can be due to the aging of the blood cell concentrate with release of their granule content, and generation of EVs, which can produce anaphylactic reactions and risk of thrombosis, but also to the presence of activated coagulation Factors in purified products, such as Factor Xia as recently identified in immunoglobulin concentrates. Characterization and measurement of contaminant products is of special usefulness during product preparation and for optimization of manufacturing processes for purified extracted products, but also for recombinant proteins. The pharmaceutical industry introduces these new methods for validating manufacturing processes, or for quality control assessments. The objective is first to warrant the full quality and safety of the lots produced, and assure the highest efficacy with the lowest risks when used in patients. For cell concentrates and fresh blood, storage conditions are critical and measurement of analytes such as EVs or Annexin V allows evaluation of quality of each individual transfused pouch. In addition to all the rules around viral and bacterial transmission risk, and immune tolerance, our available laboratory methods contribute to reducing the side effects of blood cell concentrates and derived plasma products, as well as those of the therapeutic recombinant proteins.

Platelet Integrin αIIbβ3 Inhibitor Rescues Progression of Apoptosis in Human Platelets

BACKGROUND Apoptosis plays an important role in the physiology of platelet function. We aimed to detect the effect of the platelet integrin αIIbβ3 inhibitor, tirofiban, on apoptotic events, including mitochondrial inner-membrane potential (ΔΨm), phosphatidylserine (PS) exposure on platelet surface, and the generation of reactive oxygen species (ROS), when washed platelets were stimulated with thrombin. MATERIAL AND METHODS The study included washed platelets from healthy humans, divided into 4 groups: vehicle, and tirofiban (0.05 μg/ml, 0.25 μg/ml, and 0.5 μg/ml). Platelets were pretreated with vehicle or tirofiban and incubated at 37°C with agitation for 6 h and 24 h. Before thrombin addition, the vehicle group divided into 2 equal groups. Except one vehicle group, the other 4 groups were all stimulated with thrombin (1 U/ml) for 30 min at 37°C. Using flow cytometry, we studied the DYm and PS exposure on platelet surfaces, and the generation of ROS in platelets. RESULTS We observed that at the time of 6 h and 24 h, thrombin-stimulated vehicle platelets induced significant depo-larization of ΔΨm, higher PS exposure, and increased ROS production compared with the vehicle group (P<0.01). However, the tirofiban group had significantly more recovery of DYm, PS exposure, and ROS production compared with the thrombin group (P<0.01). CONCLUSIONS The platelet integrin αIIbβ3 inhibitor, tirofiban, inhibits the depolarization of DYm, PS exposure on platelet surface, and ROS production when stimulated with thrombin. These results suggest that αIIbβ3 inhibitor inhibits the initiation of apoptosis in platelets, showing a potential clinical application of tirofiban as an apoptosis inhibitor.

Regulation of platelet membrane protein shedding in health and disease

Extracellular proteolysis of platelet plasma membrane proteins is an event that ensues platelet activation. Shedding of surface receptors such as glycoprotein (GP) Ibα, GPV and GPVI as well as externalized proteins P-selectin and CD40L releases soluble ectodomain fragments that are subsequently detectable in plasma. This results in the irreversible functional downregulation of platelet receptor-mediated adhesive interactions and the generation of biologically active fragments. In this review, we describe molecular insights into the regulation of platelet receptor and ligand shedding in health and disease. The scope of this review is specially focused on GPIbα, GPV, GPVI, P-selectin and CD40L where we: (1) describe the basic physiological regulation of expression and shedding of these proteins in hemostasis illustrate alterations in receptor expression during (2) apoptosis and (3) ex vivo storage relevant for blood banking purposes; (4) discuss considerations to be made when analyzing and interpreting shedding of platelet membrane proteins and finally; (5) collate clinical evidence that quantify these platelet proteins during disease.

Ex vivo apoptotic and autophagic influence of an estradiol analogue on platelets

Background

Platelets are known contributors to the vascularization, metastasis and growth of tumors. Upon their interaction with cancer cells they are activated resulting in degranulation and release of constituents. Since the apoptotic- and autophagic effects of 2-ethyl-3-O-sulphamoyl-estra-1,3,5(10)16-tetraene (ESE-16) has been shown to occur in vitro and this compound was designed to bind to carbonic anhydrase II (CAII), the possible occurrence of these cell death mechanisms in platelets as circulatory components, is of importance.

Methods

Scanning electron microscopy was used to assess morphological changes in platelets after exposure to ESE-16. The possible apoptotic- and autophagic effect of ESE-16 in platelets was also determined by means of flow cytometry through measurement of Annexin V-FITC, caspase 3 activity, autophagy related protein 5 levels and light chain 3-I to light chain 3-II conversion.

Results

Scanning electron microscopy revealed no changes in ESE-16-treated platelets when compared to vehicle-treated samples. Apoptosis detection by Annexin V-FITC and measurement of caspase 3 activity indicated that there was no increase in apoptosis when platelets were exposed to ESE-16. The incidence of autophagy by measurement of autophagy related protein 5 levels and light chain 3-I to light chain 3-II conversion showed that exposure to ESE-16 did not cause the incidence of autophagy in platelets.

Conclusion

This is the first ex vivo study reporting on involvement of apoptosis- and autophagy-related targets in platelets after exposure to ESE-16, warranting further investigation in platelets of cancer patients.

Quantification of adherent platelets on polymer-based biomaterials. Comparison of colorimetric and microscopic assessment

BACKGROUND

Platelet adhesion to artificial surfaces is one of the most important indicators for the thrombogenicity of implant materials. Currently, a variety of enzyme activity-based colorimetric assays or microscopy-based techniques are commonly in use to assess this characteristic. Studies about how data of colorimetric assays correlate with the image-based quantification of adherent platelets are scarce. To address this question, the present study compared two colorimetric assays (lactate dehydrogenase (LDH) and acid phosphatase (ACP)) with an image-based quantification of the density of platelets adhering on polymer-based biomaterial surfaces.

MATERIALS AND METHODS

Tri-sodium citrated whole blood was collected from apparently healthy subjects and platelet rich plasma (PRP) was prepared according to a standardized protocol. An in vitro static thrombogenicity test was applied to study platelet adhesion from PRP adjusted to 50,000 platelets per μL on three different polymers: medical grade polytetrafluoroethylene (PTFE), silicone and polyethylene terephthalate (PET). For the direct image-based approach, surface adherent platelets were fixed, fluorescently labelled and microscopically visualized. The image-based determination of platelet densities provided reference values for the comparison with data of the colorimetric assays. Correlation between standard platelet concentrations and ACP/LDH absorbance measurements were analysed to estimate accuracy and association of both parameters. ACP and LDH release from resting and ADP-stimulated platelets was studied to estimate how platelet activation influences colorimetric assay results.

RESULTS

The density of adherent platelets ranged from 15,693 ± 2,487 platelets·mm-2 (PTFE) to 423 ± 99 platelets·mm-2 (silicone) and 4,621 ± 1,427 platelets·mm-2 (PET) and differed significantly between the three polymers (ANOVA: p <  0.05). Correlation coefficients between microscopic and colorimetric determination of platelet densities ranged between r = 0.93 (LDH, p <  0.001) and r = 0.94 (ACP, p <  0.001). ACP absorbance measurements of platelet standards with different concentrations corresponded well to an ideal linear regression, while LDH data either deceeded or exceeded the expected values. The LDH release during ADP-induced platelet activation was significantly higher compared to the release of ACP.

CONCLUSION

For an adjusted platelet concentration of 50,000 platelets·μL-1, both colorimetric assays (ACP and LDH) allowed a similar accurate quantification of the mean platelet density compared to the microscopic evaluation. Better linearity of the assay standards, less variability of the results and a lower influence of platelet activation on the measurements mark the ACP assay as more suitable for the assessment of material surface adherent platelets compared to the LDH assay, particularly, if near physiological platelet concentrations are applied.

Aging of platelets stored for transfusion

A goal of platelet storage is to maintain the quality of platelets from the point of donation to the point of transfusion - to suspend the aging process. This effort is judged by clinical and laboratory measures with varying degrees of success. Recent work gives encouragement that platelets can be maintained ex vivo beyond the current 5 -7 day shelf life whilst maintaining their quality, as measured by posttransfusion recovery and survival. However, additional measures are needed to validate the development of technologies that may further reduce the aging of stored platelets, or enhance their hemostatic properties.

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.

Influence of Oxidative Stress on Stored Platelets

Platelet storage and its availability for transfusion are limited to 5-6 days. Oxidative stress (OS) is one of the causes for reduced efficacy and shelf-life of platelets. The studies on platelet storage have focused on improving the storage conditions by altering platelet storage solutions, temperature, and materials. Nevertheless, the role of OS on platelet survival during storage is still unclear. Hence, this study was conducted to investigate the influence of storage on platelets. Platelets were stored for 12 days at 22°C. OS markers such as aggregation, superoxides, reactive oxygen species, glucose, pH, lipid peroxidation, protein oxidation, and antioxidant enzymes were assessed. OS increased during storage as indicated by increments in aggregation, superoxides, pH, conjugate dienes, and superoxide dismutase and decrements in glucose and catalase. Thus, platelets could endure OS till 6 days during storage, due to the antioxidant defense system. An evident increase in OS was observed from day 8 of storage, which can diminish the platelet efficacy. The present study provides an insight into the gradual changes occurring during platelet storage. This lays the foundation towards new possibilities of employing various antioxidants as additives in storage solutions.

miR-326 targets antiapoptotic Bcl-xL and mediates apoptosis in human platelets

Platelets play crucial roles in hemostasis, thrombosis, wound healing, inflammation, angiogenesis, and tumor metastases. Because they are anucleated blood cells, platelets lack nuclear DNA, but they do contain mitochondrial DNA, which plays a key role in regulating apoptosis. Recent evidence has suggested that miRNAs are also involved in regulating gene expression and apoptosis in platelets. Our previous study showed that the expression of miR-326 increased visibly when apheresis platelets were stored in vitro. The antiapoptotic Bcl-2 family regulator Bcl-xL has been identified as a putative target of miR-326. In the present study, dual reporter luciferase assays were used to characterize the function of miR-326 in the regulation of the apoptosis of platelet cells. These assays demonstrated that miR-326 bound to the 3′-translated region of Bcl-xL. To directly assess the functional effects of miR-326 expression, levels of Bcl-xL and the apoptotic status of stored apheresis platelets were measured after transfection of miR-326 mimic or inhibitor. Results indicated that miR-326 inhibited Bcl-xL expression and induced apoptosis in stored platelets. Additionally, miR-326 inhibited Bcl-2 protein expression and enhanced Bak expression, possibly through an indirect mechanism, though there was no effect on the expression of Bax. The effect of miR-326 appeared to be limited to apoptosis, with no significant effect on platelet activation. These results provide new insight into the molecular mechanisms affecting differential platelet gene regulation, which may increase understanding of the role of platelet apoptosis in multiple diseases.

Hemostatic function of apheresis platelets stored at 4°C and 22°C

INTRODUCTION

Platelet refrigeration decreases the risk of bacterial contamination and may preserve function better than standard-of-care room temperature (RT) storage. Benefits could include lower transfusion-related complications, decreased costs, improved hemostasis in acutely bleeding patients, and extended shelf life. In this study, we compared the effects of 22°C and 4°C storage on the functional and activation status of apheresis platelets.

METHODS

Apheresis platelets (n = 5 per group) were stored for 5 days at 22°C with agitation (RT) versus at 4°C with agitation (4°C + AG) and without (4°C). Measurements included platelet counts, mean platelet volume, blood gas analytes, aggregation response, thromboelastography, thromboxane B2 and soluble CD40 ligand release, activation markers, and microparticle formation.

RESULTS

Sample pH levels were within acceptable limits for storage products (pH 6.2-7.4). Platelet glucose metabolism (P < 0.05), aggregation response (adenosine diphosphate: RT 0; 4°C + AG 5.0 ± 0.8; 4°C 5.6 ± 0.9; P < 0.05), and clot strength (maximum amplitude: RT 58 ± 2; 4°C + AG 63 ± 2; 4°C 67 ± 2; P < 0.05) were better preserved at 4°C compared with RT storage. Refrigerated samples were more activated compared with RT (P < 0.05), although thromboxane B2 (P < 0.05) and soluble CD40 ligand release (P < 0.05) were higher at RT. Agitation did not improve the quality of 4°C-stored samples.

CONCLUSIONS

Apheresis platelets stored at 4°C maintain more viable metabolic characteristics, are hemostatically more effective, and release fewer proinflammatory mediators than apheresis platelets stored at RT over 5 days. Given the superior bacteriologic safety of refrigerated products, these data suggest that cold-stored platelets may improve outcomes for acutely bleeding patients.

Effect of cold storage on shear-induced platelet aggregation and clot strength

BACKGROUND

Platelets (PLTs) participate in hemostasis and save lives following trauma. PLTs for transfusion are maintained at room temperature (RT, 22°C), limiting viability to 5 days because of metabolic compromise and high risk of bacterial contamination. RT storage may result in weaker clots, delaying hemorrhage control, whereas cold storage (4°C) could permit longer PLT shelf life and result in a more hemostatic product. In this study, we characterized the effect of storage temperature on shear-induced PLT aggregation, clot formation, and strength.

METHODS

PLTs obtained from phlebotomized blood or by apheresis were stored at RT or 4°C for 5 days, and PLT aggregation and clot strength were assessed at 37°C. We studied PLT aggregation at steady and complex patterns of shear rates (500-2,500 per second) by flow cytometry, and the kinetics of clot formation and strength were measured using turbidity and dynamic mechanical analysis, respectively.

RESULTS

PLT aggregation was higher in 4°C-stored samples on Day 5 compared with fresh or RT-stored samples at all shear rates tested (fresh vs. 4°C and RT vs. 4°C, p < 0.05). PLTs stored at 4°C for 5 days formed significantly stronger clots compared with fresh or RT-stored samples as quantified by turbidity and elastic moduli measurements (fresh vs. 4°C and RT vs. 4°C, p < 0.05).

CONCLUSION

Our results show that cold-stored PLTs are more responsive to aggregation stimuli and form stronger clots, presumably because of thicker fibrin strands. These data suggest that the superior functionality of cold-stored PLTs may support faster hemostasis for acutely bleeding in trauma patients compared with RT-stored PLTs.

Separation of in-vitro-derived megakaryocytes and platelets using spinning-membrane filtration

In-vitro-derived platelets (PLTs) could potentially overcome problems associated with donated PLTs, including contamination and alloimmunization. Although several groups have produced functional PLTs from stem cells in vitro, the challenge of developing this technology to yield transfusable PLT units has yet to be addressed. The asynchronous nature of in vitro PLT generation makes a single harvest point infeasible for collecting PLTs as soon as they are formed. The current standard of performing manual centrifugations to separate PLTs from nucleated cells at multiple points during culture is labor-intensive, imprecise, and difficult to standardize in accordance with current Good Manufacturing Practices (cGMP). In an effort to develop a more effective method, we adapted a commercially-available, spinning-membrane filtration device to separate in-vitro-derived PLTs from nucleated cells and recover immature megakaryocytes (MKs), the precursor cells to PLTs, for continued culture. Processing a mixture of in-vitro-derived MKs and PLTs on the adapted device yielded a pure PLT population and did not induce PLT pre-activation. MKs recovered from the separation process were unaffected with respect to viability and ploidy, and were able to generate PLTs after reseeding in culture. Being able to efficiently harvest in-vitro-derived PLTs brings this technology one step closer to clinical relevance.

p38MAPK is involved in apoptosis development in apheresis platelet concentrates after riboflavin and ultraviolet light treatment

BACKGROUND

Pathogen inactivation (PI) accelerates the platelet (PLT) storage lesion, including apoptotic-like changes. Proteomic studies have shown that phosphorylation levels of several kinases increase in PLTs after riboflavin and UV light (RF-PI) treatment. Inhibition of p38MAPK improved in vitro PLT quality, but the biochemical basis of this kinase's contribution to PLT damage requires further analysis.

STUDY DESIGN AND METHODS

In a pool-and-split design, apheresis PLT concentrates were either treated or kept untreated with or without selected kinase inhibitors. Samples were analyzed throughout 7 days of storage, monitoring in vitro quality variables including phosphatidylserine exposure, degranulation, and glucose metabolism. Changes in the protein expression of Bax, Bak, and Bcl-xL and the activities of caspase-3 and -9 were determined by immunoblot analysis and flow cytometry, respectively.

RESULTS

The expression levels of the proapoptotic proteins Bax and Bak, but not the antiapoptotic protein Bcl-xL, were significantly increased after the RF-PI treatment. This trend was reversed in the presence of p38MAPK inhibitor SB203580. As a result of increasing proapoptotic protein levels, caspase-3 and -9 activities were significantly increased in RF-PI treatment during storage compared with control (p < 0.05). Similarly, p38MAPK inhibition significantly reduced these caspase activities compared with vehicle control after RF-PI treatment (p < 0.05).

CONCLUSION

These findings revealed that p38MAPK is involved in signaling leading to apoptosis triggered by RF-PI. Elucidation of the biochemical processes influenced by PI is a necessary step in the development of strategies to improve the PLT quality and ameliorate the negative effects of PI treatment.

Enhanced platelet apoptosis in chronic uremic patients

BACKGROUND

There is a paucity of research on platelet apoptosis and its contribution to platelet dysfunction in uremic patients. The present study sought to analyze platelets apoptosis in uremic patients who underwent different dialysis modalities.

METHODS

Sixteen chronic uremic patients (5 on hemodialysis, 6 on peritoneal dialysis and 5 on non-dialysis) and 16 controls were studied. Platelet-rich plasma was detected for apoptotic events including depolarization of mitochondrial inner membrane potential (ΔΨm), phosphatidylserine (PS) exposure, activation of caspases-3 and Bcl-2 family proteins variations by Flow Cytometry or by Western-Blot. Washed normal platelets were incubated with normal or uremic platelet poor plasma and then were detected apoptotic events. Platelets function was assessed by ristocetin induced aggregative function test.

RESULTS

Compared to controls, uremic platelets demonstrated greater apoptosis for the ΔΨm depolarization (43.48 ± 9.58 vs. 52.76 ± 15.36, p = 0.005) as well as PS exposure (1.36 ± 0.51 vs. 0.99 ± 0.27, p < 0.001). There was no significant difference among different treatment groups (for the ΔΨm depolarization f = 0.16, p = 0.85; for the PS exposure f = 1.06, p = 0.36). Western Blot analyses showed caspase-3 activation and pro-apoptotic Bcl-2 family proteins expression. Platelets exposed to uremic plasma exhibited distinct apoptosis phenomena. Ristocetin induced platelet aggregation was markedly diminished in uremic patients and treated platelets.

CONCLUSIONS

These findings indicate that platelets are incurred apoptosis in uremia patients. Uremic plasma accelerates apoptosis of normal platelets, resulting in a dysfunctional pattern of platelets in uremia. Uremic platelets apoptosis has no relationship with dialysis modality.

Expansion of the neonatal platelet mass is achieved via an extension of platelet lifespan

The fetal/neonatal hematopoietic system must generate enough blood cells to meet the demands of rapid growth. This unique challenge might underlie the high incidence of thrombocytopenia among preterm neonates. In this study, neonatal platelet production and turnover were investigated in newborn mice. Based on a combination of blood volume expansion and increasing platelet counts, the platelet mass increased sevenfold during the first 2 weeks of murine life, a time during which thrombopoiesis shifted from liver to bone marrow. Studies applying in vivo biotinylation and mathematical modeling showed that newborn and adult mice had similar platelet production rates, but neonatal platelets survived 1 day longer in circulation. This prolonged lifespan fully accounted for the rise in platelet counts observed during the second week of murine postnatal life. A study of pro-apoptotic and anti-apoptotic Bcl-2 family proteins showed that neonatal platelets had higher levels of the anti-apoptotic protein Bcl-2 and were more resistant to apoptosis induced by the Bcl-2/Bcl-xL inhibitor ABT-737 than adult platelets. However, genetic ablation or pharmacologic inhibition of Bcl-2 alone did not shorten neonatal platelet survival or reduce platelet counts in newborn mice, indicating the existence of redundant or alternative mechanisms mediating the prolonged lifespan of neonatal 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.

Brodde M. State of the art in platelet function testing

Platelets perform many functions in hemostasis but also in other areas of physiology and pathology. Therefore, it is obvious that many different function tests have been developed, each one conceived and standardized for a special purpose. This review will summarize the different fields in which platelet function testing is currently in use; diagnostics of patients with bleeding disorders, monitoring patients' response to anti-platelet therapy, monitoring in transfusion medicine (blood donors, platelet concentrates, and after transfusion), and monitoring in perioperative medicine to predict bleeding tendency. The second part of the review outlines different methods for platelet function testing, spanning bleeding time, and platelet counting as well as determining platelet adhesion, platelet secretion, platelet aggregation, platelet morphology, platelet signal transduction, platelet procoagulant activity, platelet apoptosis, platelet proteomics, and molecular biology.

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.

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.

P2Y12 protects platelets from apoptosis via PI3k-dependent Bak/Bax inactivation

BACKGROUND

Platelet ADP receptor P2Y(12) is well studied and recognized as a key player in platelet activation, hemostasis and thrombosis. However, the role of P2Y(12) in platelet apoptosis remains unknown.

OBJECTIVES

To evaluate the role of the P2Y(12) receptor in platelet apoptosis.

METHODS

We used flow cytometry and Western blotting to assess apoptotic events in platelets treated with ABT-737 or ABT-263, and stored at 37°C, combined with P2Y(12) receptor antagonists or P2Y(12) -deficient mice.

RESULTS

 P2Y(12) activation attenuated apoptosis induced by ABT-737 in human and mouse platelets in vitro, evidenced by reduced phosphatidylserine (PS) exposure, diminished depolarization of mitochondrial inner transmembrane potential (ΔΨm) and decreased caspase-3 activation. Through increasing the phosphorylation level of Akt and Bad, and changing the interaction between different Bcl-2 family proteins, P2Y(12) activation inactivated Bak/Bax. This antiapoptotic effect could be abolished by P2Y(12) antagonism or PI3K inhibition. We also observed the antiapoptotic effect of P2Y(12) activation in platelets stored at 37°C. P2Y(12) activation improved the impaired activation responses of apoptotic platelets stressed by ABT-737. In platelets from mice dosed with ABT-263 in vivo, clopidogrel or deficiency of P2Y(12) receptor enhanced apoptosis along with increased Bak/Bax activation.

CONCLUSIONS

This study demonstrates that P2Y(12) activation protects platelets from apoptosis via PI3k-dependent Bak/Bax inactivation, which may be physiologically important to counter the proapoptotic challenge. Our findings that P2Y(12) blockade exaggerates platelet apoptosis induced by ABT-263 (Navitoclax) also imply a novel drug interaction of ABT-263 and P2Y(12) antagonists.

Phosphatidylserine-expressing cell by-products in transfusion: A pro-inflammatory or an anti-inflammatory effect?

Labile blood products contain phosphatidylserine-expressing cell dusts, including apoptotic cells and microparticles. These cell by-products are produced during blood product process or storage and derived from the cells of interest that exert a therapeutic effect (red blood cells or platelets). Alternatively, phosphatidylserine-expressing cell dusts may also derived from contaminating cells, such as leukocytes, or may be already present in plasma, such as platelet-derived microparticles. These cell by-products present in labile blood products can be responsible for transfusion-induced immunomodulation leading to either transfusion-related acute lung injury (TRALI) or increased occurrence of post-transfusion infections or cancer relapse. In this review, we report data from the literature and our laboratory dealing with interactions between antigen-presenting cells and phosphatidylserine-expressing cell dusts, including apoptotic leukocytes and blood cell-derived microparticles. Then, we discuss how these phosphatidylserine-expressing cell by-products may influence transfusion.