Apoptotic processes in megakaryocytes and platelets

Study of platelet kinetics in immune thrombocytopenia to predict splenectomy response

Despite the efficacy of splenectomy for chronic immune thrombocytopenia (ITP), its considerable failure rate and its possible related complications prove the need for further research into potential predictors of response. The platelet sequestration site determined by 111 In-labelled autologous platelet scintigraphy has been proposed to predict splenectomy outcome, but without standardisation in clinical practice. Here, we conducted a single-centre study by analysing a cohort of splenectomised patients with ITP in whom 111 In-scintigraphy was performed at La Paz University Hospital in Madrid to evaluate the predictive value of the platelet kinetic studies. We also studied other factors that could impact the splenectomy outcome, such as patient and platelet characteristics. A total of 51 patients were splenectomised, and 82.3% responded. The splenic sequestration pattern predicted a higher rate of complete response up to 12 months after splenectomy (p = 0.005), with 90% sensitivity and 77% specificity. Neither age, comorbidities, therapy lines nor previous response to them showed any association with response. Results from the platelet characteristics analysis revealed a significant loss of sialic acid in platelets from the non-responding patients compared with those who maintained a response (p = 0.0017). Our findings highlight the value of splenic sequestration as an independent predictor of splenectomy response.

Apoptosis in megakaryocytes: Safeguard and threat for thrombopoiesis

Platelets, generated from precursor megakaryocytes (MKs), are central mediators of hemostasis and thrombosis. The process of thrombopoiesis is extremely complex, regulated by multiple factors, and related to many cellular events including apoptosis. However, the role of apoptosis in thrombopoiesis has been controversial for many years. Some researchers believe that apoptosis is an ally of thrombopoiesis and platelets production is apoptosis-dependent, while others have suggested that apoptosis is dispensable for thrombopoiesis, and is even inhibited during this process. In this review, we will focus on this conflict, discuss the relationship between megakaryocytopoiesis, thrombopoiesis and apoptosis. In addition, we also consider why such a vast number of studies draw opposite conclusions of the role of apoptosis in thrombopoiesis, and try to figure out the truth behind the mystery. This review provides more comprehensive insights into the relationship between megakaryocytopoiesis, thrombopoiesis, and apoptosis and finds some clues for the possible pathological mechanisms of platelet disorders caused by abnormal apoptosis.

De novo lipogenesis is essential for platelet production in humans

Acetyl-CoA carboxylase (ACC) catalyses the first step of de novo lipogenesis (DNL). Pharmacologic inhibition of ACC has been of interest for therapeutic intervention in a wide range of diseases. We demonstrate here that ACC and DNL are essential for platelet production in humans and monkeys, but in not rodents or dogs. During clinical evaluation of a systemically distributed ACC inhibitor, unexpected dose-dependent reductions in platelet count were observed. While platelet count reductions were not observed in rat and dog toxicology studies, subsequent studies in cynomolgus monkeys recapitulated these platelet count reductions with a similar concentration response to that in humans. These studies, along with ex vivo human megakaryocyte maturation studies, demonstrate that platelet lowering is a consequence of DNL inhibition likely to result in impaired megakaryocyte demarcation membrane formation. These observations demonstrate that while DNL is a minor quantitative contributor to global lipid balance in humans, DNL is essential to specific lipid pools of physiological importance.

Effect of YAP/TAZ on megakaryocyte differentiation and platelet production

Platelet transfusion is required for life-threatening thrombocytopenic bleeding, and single donor platelet concentrate is the ideal transfusion product. However, due to the inadequate number of donors that can donate a large volume of platelets, in vitro platelets production could be an alternative. We developed an in vitro production system designed to increase the platelet production yield from cultured cells. Previously, we reported that depletion of a Hippo pathway core kinase (LATS1/2) inhibited platelet production from cultured megakaryocytes. In the present study, we further investigated the role of the Hippo pathway in megakaryocyte proliferation and platelet production by focusing on the role of its effector proteins (YAP and TAZ), which are down-stream targets of LATS1/2 kinase. We found that YAP plays an essential role in megakaryoblastic cell proliferation, maturation, and platelet production, while TAZ showed minor effect. Knockdown of YAP, either by genetic manipulation or pharmaceutical molecule, significantly increased caspase-3-mediated apoptosis in cultured megakaryocytes, and increased platelet production as opposed to overexpressing YAP. We, therefore, demonstrate a paradigm for the regulation of megakaryocyte development and platelet production via the Hippo signaling pathway, and suggest the potential use of an FDA-approved drug to induce higher platelet production in cultured cells.

Transcriptomic analysis of patients with immune thrombocytopenia treated with eltrombopag

In the last years, the use of thrombopoietin receptor agonists (TPO-RA), eltrombopag and romiplostim, has improved the management of immune thrombocytopenia (ITP). Moreover, eltrombopag is also active in patients with aplastic anemia and myelodysplastic syndrome. However, their mechanisms of action and signaling pathways still remain controversial. In order to gain insight into the mechanisms underlying eltrombopag therapy, a gene expression profile (GEP) analysis in patients treated with this drug was carried out. Fourteen patients with chronic ITP were studied by means of microarrays before and during eltrombopag treatment. Median age was 78 years (range, 35-87 years); median baseline platelet count was 14 × 10⁹/L (range, 2-68 × 10⁹/L). Ten patients responded to the therapy, two cases relapsed after an initial response and the remaining two were refractory to the therapy. Eltrombopag induced relevant changes in the hematopoiesis, platelet activation and degranulation, as well as in megakaryocyte differentiation, with overexpression of some transcription factors and the genes PPBP, ITGB3, ITGA2B, F13A1, F13A1, MYL9 and ITGA2B. In addition, GP1BA, PF4, ITGA2B, MYL9, HIST1H4H and HIST1H2BH, genes regulated by RUNX1 were also significantly enriched after eltrombopag therapy. Furthermore, in non-responder patients, an overexpression of Bcl-X gene and genes involved in erythropoiesis, such as SLC4A1 and SLC25A39, was also observed. To conclude, overexpression in genes involved in megakaryopoiesis, platelet adhesion, degranulation and aggregation was observed in patients treated with eltrombopag. Moreover, an important role regarding heme metabolism was also present in non-responder patients.

Endoplasmic reticulum stress induced apoptosis and caspase activation is mediated through mitochondria during megakaryocyte differentiation

Megakaryocytopoiesis involves the process of the development of hematopoietic stem cells into megakaryocytes (MKs), which are the specialized cells responsible for the production of blood platelets. Platelets are one of the crucial factors for hemostasis and thrombosis. In terminally differentiated MKs, many molecular process such as caspase activation and a massive cytoskeletal rearrangement drive the formation of cytoplasmic extensions called proplatelets. These cytoplasmic extensions packed with granules and organelles are then released from the bone marrow into the blood circulation as platelets. Classically, caspase activation is associated with apoptosis and recent reports suggest their involvement in cell differentiation and maturation. There is no clear evidence about the stimulus for caspase activation during megakaryocyte development. In the current study, we attempted to understand the importance of endoplasmic reticulum stress in the caspase activation during megakaryocyte maturation. We used human megakaryoblstic cell line (Dami cells) as an experimental model. We used PMA (Phorbol 12-myristate 13 acetate) to induce megakaryocytic differentiation to understand the involvement of ER stress and caspase activation during MK maturation. Further, we used Thapsigargin, a non-competitive inhibitor of the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) as a positive control to induce ER stress. We observed larger and adherent cells with the increased expression of megakaryocytic markers (CD41 and CD61) and UPR markers in PMA or Thapsigargin treated cells as compared to control. Also, Thapsigargin treatment induced increased caspase activity and PARP cleavage. The increased expression of megakaryocyte maturation markers alongside with ER stress and caspase activation suggests the importance of ER stress in caspase activation during MK maturation.

Induction of differentiation of human stem cells ex vivo: Toward large-scale platelet production

Platelet transfusion is one of the most reliable strategies to cure patients suffering from thrombocytopenia or platelet dysfunction. With the increasing demand for transfusion, however, there is an undersupply of donors to provide the platelet source. Thus, scientists have sought to design methods for deriving clinical-scale platelets ex vivo. Although there has been considerable success ex vivo in the generation of transformative platelets produced by human stem cells (SCs), the platelet yields achieved using these strategies have not been adequate for clinical application. In this review, we provide an overview of the developmental process of megakaryocytes and the production of platelets in vivo and ex vivo, recapitulate the key advances in the production of SC-derived platelets using several SC sources, and discuss some strategies that apply three-dimensional bioreactor devices and biochemical factors synergistically to improve the generation of large-scale platelets for use in future biomedical and clinical settings.

Fatal dysfunction and disintegration of thrombin-stimulated platelets

Platelets play a key role in the formation of hemostatic clots and obstructive thrombi as well as in other biological processes. In response to physiological stimulants, including thrombin, platelets change shape, express adhesive molecules, aggregate, and secrete bioactive substances, but their subsequent fate is largely unknown. Here we examined late-stage structural, metabolic, and functional consequences of thrombin-induced platelet activation. Using a combination of confocal microscopy, scanning and transmission electron microscopy, flow cytometry, biochemical and biomechanical measurements, we showed that thrombin-induced activation is followed by time-dependent platelet dysfunction and disintegration. After ~30 minutes of incubation with thrombin, unlike with collagen or ADP, human platelets disintegrated into cellular fragments containing organelles, such as mitochondria, glycogen granules, and vacuoles. This platelet fragmentation was preceded by Ca²⁺ influx, integrin α_IIbβ₃ activation and phosphatidylserine exposure (activation phase), followed by mitochondrial depolarization, generation of reactive oxygen species, metabolic ATP depletion and impairment of platelet contractility along with dramatic cytoskeletal rearrangements, concomitant with platelet disintegration (death phase). Coincidentally with the platelet fragmentation, thrombin caused calpain activation but not activation of caspases 3 and 7. Our findings indicate that the late functional and structural damage of thrombin-activated platelets comprise a calpain-dependent platelet death pathway that shares some similarities with the programmed death of nucleated cells, but is unique to platelets, therefore representing a special form of cellular destruction. Fragmentation of activated platelets suggests that there is an underappreciated pathway of enhanced elimination of platelets from the circulation in (pro)thrombotic conditions once these cells have performed their functions.

How to Avoid False-Negative and False-Positive Diagnoses of Platelet Apoptosis: Illustrative Experimental and Clinically Relevant Cases

Platelets may selectively execute apoptosis (PL-Apo), activation (PL-Act), and both or no responses when exposed to different chemical agents, shear stresses, and stored under blood banking conditions. Appropriate diagnosis of PL-Apo is an important issue of platelet physiology investigations. However, in diagnosing PL-Apo, there is a risk of a false-negative or false-positive diagnosis. The goal of the current review is to present recommendations that may help to avoid incorrect PL-Apo diagnosis. Analyzing reported studies, we recommend (1) using platelet-rich plasma rather than isolated platelets to minimize artificial stimulation of PL-Apo during platelet isolation, (2) using established optimal conditions for stimulation of PL-Apo and/or PL-Act, (3) using a panel of PL-Apo and PL-Act markers, and (4) appropriate positive and negative controls for quantification of PL-Apo and PL-Act responses.

Dopamine induces platelet production from megakaryocytes via oxidative stress-mediated signaling pathways

Dopamine (DA), a catecholamine neurotransmitter, is known to for its diverse roles on hematopoiesis, yet its function in thrombopoiesis remains poorly understood. This study shows that DA stimulation can directly induce platelet production from megakaryocytes (MKs) in the final stages of thrombopoiesis via a reactive oxygen species (ROS)-dependent pathway. The mechanism was suggested by the results that DA treatment could significantly elevate the ROS levels in MKs, and time-dependently activate oxidative stress-mediated signaling, including p38 mitogen-activated protein kinase, c-Jun NH2-terminal kinase, and caspase-3 signaling pathways, while the antioxidants N-acetylcysteine and L-glutathione could effectively inhibit the activation of these signaling pathways, as well as the ROS increase and platelet production triggered by DA. Therefore, our data revealed that the direct role and mechanism of DA in thrombopoiesis, which provides new insights into the function recognition of DA in hematopoiesis.

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.

Dysregulation of the intrinsic apoptotic pathway mediates megakaryocytic hyperplasia in myeloproliferative neoplasms

AIMS

Megakaryocyte expansion in myeloproliferative neoplasms (MPNs) is due to uncontrolled proliferation accompanied by dysregulation of proapoptotic and antiapoptotic mechanisms. Here we have investigated the intrinsic and extrinsic apoptotic pathways of megakaryocytes in human MPNs to further define the mechanisms involved.

METHODS

The megakaryocytic expression of proapoptotic caspase-8, caspase-9, Diablo, p53 and antiapoptotic survivin proteins was investigated in bone marrow specimens of the MPNs (n=145) and controls (n=15) using immunohistochemistry. The megakaryocyte percentage positivity was assessed by light microscopy and correlated with the MPN entity, JAK2V617F/CALR mutation status and platelet count.

RESULTS

The proportion of megakaryocytes in the MPNs expressing caspase-8, caspase-9, Diablo, survivin and p53 was significantly greater than controls. A greater proportion of myeloproliferative megakaryocytes expressed survivin relative to its reciprocal inhibitor, Diablo. Differences were seen between myelofibrosis, polycythaemia vera and essential thrombocythaemia for caspase-9 and p53. CALR-mutated cases had greater megakaryocyte p53 positivity compared to those with the JAK2V617F mutation. Proapoptotic caspase-9 expression showed a positive correlation with platelet count, which was most marked in myelofibrosis and CALR-mutated cases.

CONCLUSIONS

Disruptions targeting the intrinsic apoptotic cascade promote megakaryocyte hyperplasia and thrombocytosis in the MPNs. There is progressive dysfunction of apoptosis as evidenced by the marked reduction in proapoptotic caspase-9 and accumulation of p53 in myelofibrosis. The dysfunction of caspase-9, which is necessary for proplatelet formation, may be the mechanism for the excess thrombocytosis associated with CALR mutations. Survivin seems to be the key protein mediating the megakaryocyte survival signature in the MPNs and is a potential therapeutic target.

Linkage between the mechanisms of thrombocytopenia and thrombopoiesis

Thrombocytopenia is defined as a status in which platelet numbers are reduced. Imbalance between the homeostatic regulation of platelet generation and destruction is 1 potential cause of thrombocytopenia. In adults, platelet generation is a 2-stage process entailing the differentiation of hematopoietic stem cells into mature megakaryocytes (MKs; known as megakaryopoiesis) and release of platelets from MKs (known as thrombopoiesis or platelet biogenesis). Until recently, information about the genetic defects responsible for congenital thrombocytopenia was only available for a few forms of the disease. However, investigations over the past 15 years have identified mutations in genes encoding >20 different proteins that are responsible for these disorders, which has advanced our understanding of megakaryopoiesis and thrombopoiesis. The underlying pathogenic mechanisms can be categorized as (1) defects in MK lineage commitment and differentiation, (2) defects in MK maturation, and (3) defect in platelet release. Using these developmental stage categories, we here update recently described mechanisms underlying megakaryopoiesis and thrombopoiesis and discuss the association between platelet generation systems and thrombocytopenia.

Megakaryocytic hyperplasia in myeloproliferative neoplasms is driven by disordered proliferative, apoptotic and epigenetic mechanisms

AIMS

Myeloproliferative neoplasms (MPN) are a heterogeneous group of clonal proliferative bone marrow diseases characterised by extensive megakaryocytic hyperplasia and morphological atypia. Despite knowledge of genomic defects, the pathobiological processes driving these megakaryocytic abnormalities in MPN remain poorly explained. We have explored the proliferative, apoptotic and epigenetic profiles of megakaryocytes in human MPN.

METHODS

Immunohistochemical staining was performed on bone marrow trephine biopsies of 81 MPN (with and without JAK2(V617F) and CALR mutations) and 15 normal controls to assess the megakaryocytic expression of biomarkers associated with proliferation (Ki67), apoptosis (Bcl-XL, BNIP-3) and epigenetic regulation (EZH2, SUZ12).

RESULTS

Myeloproliferative megakaryocytes showed significantly greater expression of proliferative Ki67 and anti-apoptotic Bcl-XL, reduced pro-apoptotic BNIP-3 and increased SUZ12 compared with controls. In essential thrombocythaemia, large-giant megakaryocytes with hyperlobated nuclei showed a trend towards a proliferative signature. In contrast, myelofibrotic megakaryocytes with condensed nuclear chromatin, and cases with CALR mutations, had significant reductions in pro-apoptotic BNIP-3.

CONCLUSIONS

Uncontrolled megakaryocytic expansion in MPN results from a combination of increased proliferation, attenuated apoptosis and defective epigenetic regulation with CALR mutations favouring apoptotic failure. The higher platelet counts reported to be seen in MPN with CALR mutations may be due to greater dysregulation of megakaryocyte apoptosis.

Recruitment of CD8(+) T cells into bone marrow might explain the suppression of megakaryocyte apoptosis through high expression of CX3CR1(+) in prolonged isolated thrombocytopenia after allogeneic hematopoietic stem cell transplantation

Prolonged isolated thrombocytopenia is a common complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT), which is associated with a poor prognosis. This study aimed to investigate the pathogenesis of prolonged isolated thrombocytopenia (PT). We analysed the expression of CX3CR1 on CD4 and CD8 T cells in bone marrow (BM) and peripheral blood (PB) at +90 days from allo-HSCT recipients with or without PT by flow cytometry analyses. We then determined the megakaryocytes ploidy distributions, apoptosis rate and Fas expression of recipients with or without PT in vitro directly or after depleting CD8(+) T cells or adding purified autologous CD8(+) T cells to CD8(+) T-dep MNCs. We found that the percentage of CD8(+) T cells in BM was higher in the patients with PT than in the controls. The elevated expression of the CX3CR1 was associated with PT. There was a marked increase in the percentage of low ploidy megakaryocytes in the recipients with PT. The depletion of CD8(+) T cells increased the apoptosis of megakaryocytes and decreased the expression of Fas, which could be corrected by re-adding purified autologous CD8(+) T cells. The increase of CD8(+) T cells and CD8(+)/CX3CR1(+) T cells in BM at +90 days were independent risk factors for PT according to multivariate analysis. Our data implied that the recruitment of CD8(+) T cells into BM might explain the suppression of megakaryocyte apoptosis through the elevated expression of CX3CR1(+) in PT after allo-HSCT. CX3CR1 might be a novel treatment target in recipients with PT.

Actin inhibition increases megakaryocyte proplatelet formation through an apoptosis-dependent mechanism

Background

Megakaryocytes assemble and release platelets through the extension of proplatelet processes, which are cytoplasmic extensions that extrude from the megakaryocyte and form platelets at their tips. Proplatelet formation and platelet release are complex processes that require a combination of structural rearrangements. While the signals that trigger the initiation of proplatelet formation process are not completely understood, it has been shown that inhibition of cytoskeletal signaling in mature megakaryocytes induces proplatelet formation. Megakaryocyte apoptosis may also be involved in initiation of proplatelet extension, although this is controversial. This study inquires whether the proplatelet production induced by cytoskeletal signaling inhibition is dependent on activation of apoptosis.

Methods

Megakaryocytes derived from human umbilical cord blood CD34+ cells were treated with the actin polymerization inhibitor latrunculin and their ploidy and proplatelet formation were quantitated. Apoptosis activation was analyzed by flow cytometry and luminescence assays. Caspase activity was inhibited by two compounds, ZVAD and QVD. Expression levels of pro-survival and pro-apoptosis genes were measured by quantitative RT-PCR. Protein levels of Bcl-XL, Bax and Bak were measured by western blot. Cell ultrastructure was analyzed by electron microscopy.

Results

Actin inhibition resulted in increased ploidy and increased proplatelet formation in cultured umbilical cord blood-derived megakaryocytes. Actin inhibition activated apoptosis in the cultured cells. The effects of actin inhibition on proplatelet formation were blocked by caspase inhibition. Increased expression of both pro-apoptotic and pro-survival genes was observed. Pro-survival protein (Bcl-x_L) levels were increased compared to levels of pro-apoptotic proteins Bak and Bax. Despite apoptosis being activated, the megakaryocytes underwent minimal ultrastructural changes during actin inhibition.

Conclusions

We report a correlation between increased proplatelet formation and activation of apoptosis, and that the increase in proplatelet formation in response to actin inhibition is caspase dependent. These findings support a role for apoptosis in proplatelet formation in this model.

Megakaryopoiesis and platelet production: insight into hematopoietic stem cell proliferation and differentiation

Hematopoietic stem cells (HSCs) undergo successive lineage commitment steps to generate megakaryocytes (MKs) in a process referred to as megakaryopoiesis. MKs undergo a unique differentiation process involving endomitosis to eventually produce platelets. Many transcription factors participate in the regulation of this complex progress. Chemokines and other factors in the microenvironment where megakaryopoiesis and platelet production occur play vital roles in the regulation of HSC lineage commitment and MK maturation; among these factors, thrombopoietin (TPO) is the most important. Endomitosis is a vital process of MK maturation, and granules that are formed in MKs are important for platelet function. Proplatelets are firstly generated from mature MKs and then become platelets. The proplatelet production process was verified by novel studies that revealed that the mechanism is partially regulated by the invaginated membrane system (IMS), microtubules and Rho GTPases. The extracellular matrices (ECMs) and shear stress also affect and regulate the process while the mature MKs migrate from the marrow to the sub-endothelium region near the venous sinusoids leading to the release of platelets into the circulation. This review describes the entire process of megakaryopoiesis in detail, illustrates both the transcriptional and microenvironmental regulation of MKs and provides insight into platelet biogenesis.

Platelets Contribute to BBB Disruption Induced by HIV and Alcohol

The role of platelets in the neurological diseases that underlie cognitive impairment has attracted increasing attention in recent years. Multiple pathways in platelets contribute to host defenses, as well as to CNS function. In the current study, we hypothesize that the Blood Brain Barrier (BBB) is disrupted when exposed to platelets from patients with triple Co-morbidity (hazardous alcohol users+ HIV+ thrombocytopenia), compared to those with dual, single or no morbidity (HIV only, alcohol only or healthy controls).

Loss of Bak enhances lymphocytosis but does not ameliorate thrombocytopaenia in BCL-2 transgenic mice

Bax and Bak are critical effectors of apoptosis. Although both are widely expressed and usually functionally redundant, recent studies suggest that Bak has particular importance in certain cell types. Genetic and biochemical studies indicate that Bak activation is prevented primarily by Mcl-1 and Bcl-xL, whereas Bax is held in check by all pro-survival Bcl-2 homologues, including Bcl-2 itself. In this study, we have investigated whether loss of Bak or elevated Mcl-1 modulates haemopoietic abnormalities provoked by overexpression of Bcl-2. The Mcl-1 transgene had little impact, probably because the expression level was insufficient to effectively reduce Bak activation. However, loss of Bak enhanced lymphocytosis in vavP-BCL-2 transgenic mice and increased resistance of their thymocytes to some cytotoxic agents, implying that Bak-specific signals can be triggered in certain lymphoid populations. Nevertheless, lack of Bak had no significant impact on thymic abnormalities in vavP-BCL-2tg mice, which kinetic analysis suggested was due to accumulation of self-reactive thymocytes that resist deletion. Intriguingly, although Bak(-/-) mice have elevated platelet counts, Bak(-/-)vavP-BCL-2 mice, like vavP-BCL-2 littermates, were thrombocytopaenic. To clarify why, the vavP-BCL-2 platelet phenotype was scrutinised more closely. Platelet life span was found to be elevated in vavP-BCL-2 mice, which should have provoked thrombocytosis, as in Bak(-/-) mice. Analysis of bone marrow chimaeric mice suggested the low platelet phenotype was due principally to extrinsic factors. Following splenectomy, blood platelets remained lower in vavP-BCL-2 than wild-type mice. However, in Rag1(-/-) BCL-2tg mice, platelet levels were normal, implying that elevated lymphocytes are primarily responsible for BCL-2tg-induced thrombocytopaenia.

Stimulation of platelet apoptosis by peptidoglycan from Staphylococcus aureus 113

Peptidoglycan (PGN), a component of bacterial cell wall and belonging to "Microbe-Associated Molecular Patterns" (MAMP) triggers host reactions contributing to the pathophysiology of infectious disease. Host cell responses to PGN exposure include apoptosis. Bacterial infections may result in activation of blood platelets and thrombocytopenia. The present study explored, whether HPLC-purified fractions of PGNs from Staphylococcus aureus 113 triggers apoptosis of platelets. To this end platelets were exposed to PGN fractions and annexin-V binding determined to depict cell membrane scrambling, DiOC6 fluorescence to estimate depolarization of mitochondrial potential, Fluo-3AM staining for intracellular Ca(2+) activity ([Ca(2+)](i)) and immunofluorescence to quantify protein abundance of active caspase-3. As a result, a 30 min exposure to monomeric fraction (mPGN) (≥50 ng/ml) was followed by annexin-V binding, paralleled by increase of [Ca(2+)](i), mitochondrial depolarization, caspase-3 activation and integrin α(IIb)β(3) upregulation. The annexin-V binding was significantly blunted by anti-TLR-2 antibodies, in absence of extracellular Ca(2+), and by pancaspase inhibitor zVAD-FMK (1 μM). In conclusion, PGN triggers apoptosis of platelets in activation-dependent manner, characterized by mitochondrial depolarization, caspase-3 activation and cell membrane scrambling.

JAK2V617F leads to intrinsic changes in platelet formation and reactivity in a knock-in mouse model of essential thrombocythemia

The principal morbidity and mortality in patients with essential thrombocythemia (ET) and polycythemia rubra vera (PV) stems from thrombotic events. Most patients with ET/PV harbor a JAK2V617F mutation, but its role in the thrombotic diathesis remains obscure. Platelet function studies in patients are difficult to interpret because of interindividual heterogeneity, reflecting variations in the proportion of platelets derived from the malignant clone, differences in the presence of additional mutations, and the effects of medical treatments. To circumvent these issues, we have studied a JAK2V617F knock-in mouse model of ET in which all megakaryocytes and platelets express JAK2V617F at a physiological level, equivalent to that present in human ET patients. We show that, in addition to increased differentiation, JAK2V617F-positive megakaryocytes display greater migratory ability and proplatelet formation. We demonstrate in a range of assays that platelet reactivity to agonists is enhanced, with a concomitant increase in platelet aggregation in vitro and a reduced duration of bleeding in vivo. These data suggest that JAK2V617F leads to intrinsic changes in both megakaryocyte and platelet biology beyond an increase in cell number. In support of this hypothesis, we identify multiple differentially expressed genes in JAK2V617F megakaryocytes that may underlie the observed biological differences.

Advances in megakaryocytopoiesis and thrombopoiesis: from bench to bedside

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, proliferation and terminal differentiation of megakaryocytic progenitors (MK-p) and maturation of megakaryocytes (MKs) to produce functional platelets. This complex process occurs in specialized niches in the bone marrow where MKs align adjacent to vascular endothelial cells, form proplatelet projections and release platelets into the circulation. Thrombopoietin (THPO, TPO) is the primary growth factor for the MK lineage and necessary at all stages of development. THPO is constitutively produced in the liver, and binds to MPL (c-Mpl) receptor on platelets and MKs. This activates a cascade of signalling molecules, which induce transcription factors to drive MK development and thrombopoiesis. Decreased turnover rate and platelet number result in increased levels of free THPO, which induces a concentration-dependent compensatory response of marrow-MKs to enhance platelet production. Newly developed thrombopoietic agents operating via MPL receptor facilitate platelet production in thrombocytopenic states, primarily immune thrombocytopenia. Other drugs are available for attenuating malignant thrombocytosis. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease states, and the innovative drugs and therapeutic modalities to stimulate or decrease thrombopoiesis.

Stimulation of platelet apoptosis by balhimycin

Glycopeptides, such as vancomycin, are powerful antibiotics against methicillin-resistant Staphylococcus aureus. Balhimycin, a glycopeptide antibiotic isolated from Amycolatopsis balhimycina, is similarly effective as vancomycin. Side effects of vancomycin include triggering of platelet apoptosis, which is characterized by cell shrinkage and by cell membrane scrambling with phosphatidylserine exposure at the cell surface. Stimulation of apoptosis may involve increase of cytosolic Ca(2+) activity, ceramide formation, mitochondrial depolarization and/or caspase activation. An effect of balhimycin on apoptosis has, however, never been reported. The present study thus tested whether balhimycin triggers platelet apoptosis. Human blood platelets were treated with balhimycin and cell volume was estimated from forward scatter, phosphatidylserine exposure from annexin V-binding, cytosolic Ca(2+) activity from fluo-3AM fluorescence, ceramide formation utilizing antibodies, mitochondrial potential from DiOC6 fluorescence, and caspase-3 activity utilizing antibodies. As a result, a 30 min exposure to balhimycin significantly decreased cell volume (≥1 μg/ml), triggered annexin V binding (≥1 μg/ml), increased cytosolic Ca(2+) activity (≥1 μg/ml), stimulated ceramide formation (≥10 μg/ml), depolarized mitochondria (≥1 μg/ml) and activated caspase-3 (≥1 μg/ml). Cell membrane scrambling and caspase-3 activation were virtually abrogated by removal of extracellular Ca(2+). Cell membrane scrambling was not significantly blunted by pancaspase inhibition with zVAD-FMK (1μM). In conclusion, balhimycin triggers cell membrane scrambling of platelets, an effect dependent on Ca(2+), but not on activation of caspases.

In vitro effects of nitric oxide donors on apoptosis and oxidative/nitrative protein modifications in ADP-activated platelets

Nitric oxide (NO) is an important physiological signaling molecule. However, when produced in excessive amounts, NO can also have toxic effects. The aim of this study is to investigate the effects of exogenous- and endogenous-derived NO on oxidative modifications of proteins and apoptosis in activated platelets. Washed platelets were incubated with L-arginine or nitroso-glutathione (GSNO) in the presence of adenosine diphosphate (ADP). After incubation, caspase-3 activity, phosphatidylserine (PS) externalization and the potential of mitochondrial membrane as markers of apoptosis were measured. In addition, the alterations in protein carbonylation (PCO) and nitrotyrosine (NT) formation as markers of protein oxidation were examined. Platelet activation with ADP (20 µM) significantly increased PCO and NT levels and apoptotic events. After incubation with L-arginine, platelet NO production increased significantly. This L-arginine-induced increase caused decreases in formerly increased PCO and NT levels associated with ADP-induced platelet activation. Stimulation of NO production with L-arginine protected platelets from apoptosis. GSNO caused an increase in protein NT levels. Despite this change, GSNO was effective in inhibition of P-selectin expression, platelet aggregation, protein carbonylation and apoptosis. The results suggest that L-arginine and GSNO-mediated NO leads to the inhibition of key apoptotic processes including caspase-3 activation, PS exposure and low mitochondrial membrane potential in washed platelets. The inhibitory effect of platelet clearance of L-arginine and GSNO may be a novel useful therapeutic property in clinical application.

Death-defying apoptotic caspases in thrombopoiesis

In this issue of Blood, White et al provide compelling evidence that the intrinsic apoptotic caspase cascade, while required for megakaryocyte and platelet death under pathophysiologic conditions, is dispensable for normal platelet formation and function.

A plasma microRNA signature of acute lentiviral infection: biomarkers of central nervous system disease

OBJECTIVE

Plasma microRNAs (miRNAs) are modulated during disease and are emerging biomarkers; they have not been characterized in HIV infection. Using our macaque/simian immunodeficiency virus (SIV) model of HIV, we sought to identify a plasma miRNA profile of acute lentiviral infection, evaluate its relationship with known cellular and viral determinants of lentivirus-associated central nervous system (CNS) disease, and explore the potential of miRNAs to predict CNS disease.

DESIGN

Plasma samples were obtained before inoculation and 10 days after inoculation from SIV-infected macaques.

METHODS

Plasma miRNA expression profiles were determined by TaqMan low-density array for six individuals. miRNA expression was compared with levels of cytokines, virus, and plasma platelet count. miRNA results were confirmed by single miRNA-specific assays for 10 macaques. Nineteen individuals were used to validate a disease prediction test.

RESULTS

A 45-miRNA signature of acute infection (differential expression with P < 0.05 after multiple comparison correction) classified plasma as infected or not. Several differentially expressed miRNAs correlated with CNS disease-associated cytokines interleukin-6 and CCL2 and included predicted and/or validated regulators of the corresponding mRNAs. miRNAs tracked with viral load and platelet count were also predictors of CNS disease. At least six miRNAs were significantly differentially expressed in individuals with severe versus no CNS disease; in an unweighted expression test, they predicted CNS disease.

CONCLUSION

Acute-phase differential expression of plasma miRNAs predicts CNS disease and suggests that CNS damage or predisposition to disease progression begins in the earliest phase of infection. Plasma miRNAs should be investigated further as leading indicators of HIV diseases as early as acute infection.

HIV and SIV Associated Thrombocytopenia: An Expanding Role for Platelets in the Pathogenesis of HIV

Thrombocytopenia is common in HIV and SIV infection, and is often associated with disease progression. HIV and SIV-associated thrombocytopenia arise through multiple mechanisms, including decreased platelet production, increased platelet destruction due to HIV-mimetic anti-platelet antibodies, and increased use of activated platelets. Activated platelets have the potential to contribute to the pathogenesis of HIV and SIV by interacting directly with inflammatory cells and endothelium and by releasing soluble immunomodulatory cytokines.

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.