Consensus report on markers to distinguish procoagulant platelets from apoptotic platelets: communication from the Scientific and Standardization Committee of the ISTH.

BACKGROUND: Procoagulant platelets are a subpopulation of highly activated platelets that promote coagulation through surface-exposed, negatively charged phospholipids, especially phosphatidylserine. Procoagulant platelets are important for clot stabilization during hemostasis, and an increased number of these platelets is associated with thrombotic risk. There is a need for harmonization in this area since many of the markers and methods used to assess procoagulant platelets are not specific when used in isolation but are also associated with platelet apoptosis. OBJECTIVES: We initiated this project to identify a minimum set of markers and/or methods that can detect and distinguish procoagulant platelets from apoptotic platelets. METHODS: The study design involved a primary panel with 27 international experts who participated in an online survey and moderated virtual focus group meetings. Primary and secondary panel members were then invited to provide input on themes and statements generated from the focus groups. RESULTS: This led to a recommendation to use flow cytometry and a combination of the following 3 surface markers to differentiate procoagulant platelets from apoptotic platelets: P-selectin (CD62P), phosphatidylserine (recognized by annexin V), and the platelet-specific receptor GPIX (CD42a) or αIIb integrin (CD41, GPIIb). CONCLUSION: Procoagulant platelets are expected to be positive for all 3 markers, while apoptotic platelets are positive for annexin V and the platelet-specific surface receptor(s) but negative for P-selectin.

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.

Proinflammatory microenvironment promotes lymphoma progression in mice with high megakaryocyte and TPO levels.

Platelets have been shown to enhance the survival of lymphoma cell lines. However, it remains unclear whether they play a role in lymphoma. Here, we investigated the potential role of platelets and/or megakaryocytes in the progression of Eµ-myc lymphoma. Eµ-myc tumor cells were transplanted into recipient wild-type (WT) control, Mpl-/-, or TpoTg mice, which exhibited normal, low, and high platelet and megakaryocyte counts, respectively. TpoTg mice that underwent transplantation exhibited enhanced lymphoma progression with increased white blood cell (WBC) counts, spleen and lymph node weights, and enhanced liver infiltration when compared with WT mice. Conversely, tumor-bearing Mpl-/- mice had reduced WBC counts, lymph node weights, and less liver infiltration than WT mice. Using an Mpl-deficient thrombocytopenic immunocompromised mouse model, our results were confirmed using the human non-Hodgkin lymphoma GRANTA cell line. Although we found that platelets and platelet-released molecules supported Eµ-myc tumor cell survival in vitro, pharmacological inhibition of platelet function or anticoagulation in WT mice transplanted with Eµ-myc did not improve disease outcome. Furthermore, transient platelet depletion or sustained Bcl-xL-dependent thrombocytopenia did not alter lymphoma progression. Cytokine analysis of the bone marrow fluid microenvironment revealed increased levels of the proinflammatory molecule interleukin 1 in TpoTg mice, whereas these levels were lower in Mpl-/- mice. Moreover, RNA sequencing of blood-resident Eµ-myc lymphoma cells from TpoTg and WT mice after tumor transplantation revealed the upregulation of hallmark gene sets associated with an inflammatory response in TpoTg mice. We propose that the proinflammatory microenvironment in TpoTg mice promotes lymphoma progression.

Deletion of Grin1 in mouse megakaryocytes reveals NMDA receptor role in platelet function and proplatelet formation.

The process of proplatelet formation (PPF) requires coordinated interaction between megakaryocytes (MKs) and the extracellular matrix (ECM), followed by a dynamic reorganization of the actin and microtubule cytoskeleton. Localized fluxes of intracellular calcium ions (Ca2+) facilitate MK-ECM interaction and PPF. Glutamate-gated N-methyl-D-aspartate receptor (NMDAR) is highly permeable to Ca2+. NMDAR antagonists inhibit MK maturation ex vivo; however, there are no in vivo data. Using the Cre-loxP system, we generated a platelet lineage-specific knockout mouse model of reduced NMDAR function in MKs and platelets (Pf4-Grin1-/- mice). Effects of NMDAR deletion were examined using well-established assays of platelet function and production in vivo and ex vivo. We found that Pf4-Grin1-/- mice had defects in megakaryopoiesis, thrombopoiesis, and platelet function, which manifested as reduced platelet counts, lower rates of platelet production in the immune model of thrombocytopenia, and prolonged tail bleeding time. Platelet activation was impaired to a range of agonists associated with reduced Ca2+ responses, including metabotropic like, and defective platelet spreading. MKs showed reduced colony and proplatelet formation. Impaired reorganization of intracellular F-actin and α-tubulin was identified as the main cause of reduced platelet function and production. Pf4-Grin1-/- MKs also had lower levels of transcripts encoding crucial ECM elements and enzymes, suggesting NMDAR signaling is involved in ECM remodeling. In summary, we provide the first genetic evidence that NMDAR plays an active role in platelet function and production. NMDAR regulates PPF through a mechanism that involves MK-ECM interaction and cytoskeletal reorganization. Our results suggest that NMDAR helps guide PPF in vivo.

The RNA-binding protein SRSF3 has an essential role in megakaryocyte maturation and platelet production.

RNA processing is increasingly recognized as a critical control point in the regulation of different hematopoietic lineages including megakaryocytes responsible for the production of platelets. Platelets are anucleate cytoplasts that contain a rich repertoire of RNAs encoding proteins with essential platelet functions derived from the parent megakaryocyte. It is largely unknown how RNA binding proteins contribute to the development and functions of megakaryocytes and platelets. We show that serine-arginine-rich splicing factor 3 (SRSF3) is essential for megakaryocyte maturation and generation of functional platelets. Megakaryocyte-specific deletion of Srsf3 in mice led to macrothrombocytopenia characterized by megakaryocyte maturation arrest, dramatically reduced platelet counts, and abnormally large functionally compromised platelets. SRSF3 deficient megakaryocytes failed to reprogram their transcriptome during maturation and to load platelets with RNAs required for normal platelet function. SRSF3 depletion led to nuclear accumulation of megakaryocyte mRNAs, demonstrating that SRSF3 deploys similar RNA regulatory mechanisms in megakaryocytes as in other cell types. Our study further suggests that SRSF3 plays a role in sorting cytoplasmic megakaryocyte RNAs into platelets and demonstrates how SRSF3-mediated RNA processing forms a central part of megakaryocyte gene regulation. Understanding SRSF3 functions in megakaryocytes and platelets provides key insights into normal thrombopoiesis and platelet pathologies as SRSF3 RNA targets in megakaryocytes are associated with platelet diseases.

Apoptotic Ablation of Platelets Reduces Atherosclerosis in Mice With Diabetes.

OBJECTIVE: People with diabetes are at a significantly higher risk of cardiovascular disease, in part, due to accelerated atherosclerosis. Diabetic subjects have increased number of platelets that are activated, more reactive, and respond suboptimally to antiplatelet therapies. We hypothesized that reducing platelet numbers by inducing their premature apoptotic death would decrease atherosclerosis. Approach and Results: This was achieved by targeting the antiapoptotic protein Bcl-xL (B-cell lymphoma-extra large; which is essential for platelet viability) via distinct genetic and pharmacological approaches. In the former, we transplanted bone marrow from mice carrying the Tyr15 to Cys loss of function allele of Bcl-x (known as Bcl-xPlt20) or wild-type littermate controls into atherosclerotic-prone Ldlr+/- mice made diabetic with streptozotocin and fed a Western diet. Reduced Bcl-xL function in hematopoietic cells significantly decreased platelet numbers, exclusive of other hematologic changes. This led to a significant reduction in atherosclerotic lesion formation in Bcl-xPlt20 bone marrow transplanted Ldlr+/- mice. To assess the potential therapeutic relevance of reducing platelets in atherosclerosis, we next targeted Bcl-xL with a pharmacological strategy. This was achieved by low-dose administration of the BH3 (B-cell lymphoma-2 homology domain 3) mimetic, ABT-737 triweekly, in diabetic Apoe-/- mice for the final 6 weeks of a 12-week study. ABT-737 normalized platelet numbers along with platelet and leukocyte activation to that of nondiabetic controls, significantly reducing atherosclerosis while promoting a more stable plaque phenotype. CONCLUSIONS: These studies suggest that selectively reducing circulating platelets, by targeting Bcl-xL to promote platelet apoptosis, can reduce atherosclerosis and lower cardiovascular disease risk in diabetes. Graphic Abstract: A graphic abstract is available for this article.

The necroptotic cell death pathway operates in megakaryocytes, but not in platelet synthesis.

Necroptosis is a pro-inflammatory cell death program executed by the terminal effector, mixed lineage kinase domain-like (MLKL). Previous studies suggested a role for the necroptotic machinery in platelets, where loss of MLKL or its upstream regulator, RIPK3 kinase, impacted thrombosis and haemostasis. However, it remains unknown whether necroptosis operates within megakaryocytes, the progenitors of platelets, and whether necroptotic cell death might contribute to or diminish platelet production. Here, we demonstrate that megakaryocytes possess a functional necroptosis signalling cascade. Necroptosis activation leads to phosphorylation of MLKL, loss of viability and cell swelling. Analyses at steady state and post antibody-mediated thrombocytopenia revealed that platelet production was normal in the absence of MLKL, however, platelet activation and haemostasis were impaired with prolonged tail re-bleeding times. We conclude that MLKL plays a role in regulating platelet function and haemostasis and that necroptosis signalling in megakaryocytes is dispensable for platelet production.

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.

Membrane budding is a major mechanism of in vivo platelet biogenesis.

How platelets are produced by megakaryocytes in vivo remains controversial despite more than a century of investigation. Megakaryocytes readily produce proplatelet structures in vitro; however, visualization of platelet release from proplatelets in vivo has remained elusive. We show that within the native prenatal and adult environments, the frequency and rate of proplatelet formation is incompatible with the physiological demands of platelet replacement. We resolve this inconsistency by performing in-depth analysis of plasma membrane budding, a cellular process that has previously been dismissed as a source of platelet production. Our studies demonstrate that membrane budding results in the sustained release of platelets directly into the peripheral circulation during both fetal and adult life without induction of cell death or proplatelet formation. In support of this model, we demonstrate that in mice deficient for NF-E2 (the thrombopoietic master regulator), the absence of membrane budding correlates with failure of in vivo platelet production. Accordingly, we propose that membrane budding, rather than proplatelet formation, supplies the majority of the platelet biomass.

A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction.

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, MlklD139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of MlklD139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

Targeting platelets for improved outcome in KRAS-driven lung adenocarcinoma.

Elevated platelet count is associated with poor survival in certain solid cancers, including lung cancer. In addition, experimental transplantation of cancer cell lines has uncovered a role for platelets in blood-borne metastasis. These studies, however, do not account for heterogeneity between lung cancer subtypes. Subsequently, the role of platelets in the major subtypes of non-small cell lung cancer (adenocarcinoma (ADC) and squamous cell carcinoma (SqCC)) is not fully understood. We utilised an autochthonous KrasLSL-G12D/+;p53flox/flox mouse model of lung ADC together with genetic models of thrombocytopenia to interrogate the role of platelets in lung cancer growth and progression. While thrombocytopenia failed to impact primary tumour growth, in experimental metastatic models however, thrombocytopenic mice displayed significantly extended survival. Utilising a novel thrombocytopenic immunocompromised mouse, the importance of platelets in metastatic dissemination was confirmed with human KRAS-mutant ADC cell lines. Finally, retrospective analysis of a NSCLC patient cohort revealed thrombocytosis was predictive of poor survival in ADC patients with metastatic disease. Interestingly, this association was not apparent in SqCC patients. Overall, these data highlight the possibility of patient stratification using thrombocytosis as a biomarker, and indicates opportunities for potential novel treatment strategies that combine anti-platelet and lung cancer therapies.

Platelets disrupt vasculogenic mimicry by cancer cells.

Tumour vasculature supports the growth and progression of solid cancers with both angiogenesis (endothelial cell proliferation) and vasculogenic mimicry (VM, the formation of vascular structures by cancer cells themselves) predictors of poor patient outcomes. Increased circulating platelet counts also predict poor outcome for cancer patients but the influence of platelets on tumour vasculature is incompletely understood. Herein, we show with in vitro assays that platelets did not influence angiogenesis but did actively inhibit VM formation by cancer cell lines. Both platelet sized beads and the releasates from platelets were partially effective at inhibiting VM formation suggesting that direct contact maximises the effect. Platelets also promoted cancer cell invasion in vitro. B16F10 melanomas in Bcl-xPlt20/Plt20 thrombocytopenic mice showed a higher content of VM than their wildtype counterparts while angiogenesis did not differ. In a xenograft mouse model of breast cancer with low-dose aspirin to inactivate the platelets, the burden of MDA-MB-231-LM2 breast cancer cells was reduced and the gene expression profile of the cancer cells was altered; but no effect on tumour vasculature was observed. Taken together, this study provides new insights into the action of platelets on VM formation and their involvement in cancer progression.

N-methyl-d-aspartate receptor mediated calcium influx supports in vitro differentiation of normal mouse megakaryocytes but proliferation of leukemic cell lines.

BACKGROUND: N-methyl-d-aspartate receptors (NMDARs) contribute calcium influx in megakaryocytic cells but their roles remain unclear; both pro- and anti-differentiating effects have been shown in different contexts. OBJECTIVES: The aim of this study was to clarify NMDAR contribution to megakaryocytic differentiation in both normal and leukemic cells. METHODS: Meg-01, Set-2, and K-562 leukemic cell lines were differentiated using phorbol-12-myristate-13-acetate (PMA, 10 nmol L-1) or valproic acid (VPA, 500 µmol L-1). Normal megakaryocytes were grown from mouse marrow-derived hematopoietic progenitors (lineage-negative and CD41a-enriched) in the presence of thrombopoietin (30-40 nmol L-1). Marrow explants were used to monitor proplatelet formation in the native bone marrow milieu. In all culture systems, NMDARs were inhibited using memantine and MK-801 (100 µmol L-1); their effects compared against appropriate controls. RESULTS: The most striking observation from our studies was that NMDAR antagonists markedly inhibited proplatelet formation in all primary cultures employed. Proplatelets were either absent (in the presence of memantine) or short, broad and intertwined (with MK-801). Earlier steps of megakaryocytic differentiation (acquisition of CD41a and nuclear ploidy) were maintained, albeit reduced. In contrast, in leukemic Meg-01 cells, NMDAR antagonists inhibited differentiation in the presence of PMA and VPA but induced differentiation when applied by themselves. CONCLUSIONS: NMDAR-mediated calcium influx is required for normal megakaryocytic differentiation, in particular proplatelet formation. However, in leukemic cells, the main NMDAR role is to inhibit differentiation, suggesting diversion of NMDAR activity to support leukemia growth. Further elucidation of the NMDAR and calcium pathways in megakaryocytic cells may suggest novel ways to modulate abnormal megakaryopoiesis.

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

The circulating life span of blood platelets is regulated by the prosurvival protein BCL-XL It restrains the activity of BAK and BAX, the essential prodeath mediators of intrinsic apoptosis. Disabling the platelet intrinsic apoptotic pathway in mice by deleting BAK and BAX results in a doubling of platelet life span and concomitant thrombocytosis. Apoptotic platelets expose phosphatidylserine (PS) via a mechanism that is distinct from that driven by classical agonists. Whether there is any role for apoptotic PS in platelet function in vivo, however, is unclear. Apoptosis has also been associated with the platelet storage lesion (PSL), the constellation of biochemical deteriorations that occur during blood bank storage. In this study, we investigated the role of BAK/BAX-mediated apoptosis in hemostasis and thrombosis and in the development of the PSL. We show that although intrinsic apoptosis is rapidly induced during storage at 37°C, it is not detected when platelets are kept at the standard storage temperature of 22°C. Remarkably, loss of BAK and BAX did not prevent the development of the PSL at either temperature. BAK/BAX-deficient mice exhibited increased bleeding times and unstable thrombus formation. This phenotype was not caused by impaired PS exposure, but was associated with a defect in granule release from aged platelets. Strikingly, rejuvenation of BAK/BAX-deficient platelets in vivo completely rescued the observed hemostatic defects. Thus, apoptotic culling of old platelets from the bloodstream is essential to maintain a functional, hemostatically reactive platelet population. Inhibiting intrinsic apoptosis in blood banked platelets is unlikely to yield significant benefit.

Altered B-lymphopoiesis in mice with deregulated thrombopoietin signaling.

Thrombopoietin (TPO) is the master cytokine regulator of megakaryopoiesis. In addition to regulation of megakaryocyte and platelet number, TPO is important for maintaining proper hematopoietic stem cell (HSC) function. It was previously shown that a number of lymphoid genes were upregulated in HSCs from Tpo -/- mice. We investigated if absent or enhanced TPO signaling would influence normal B-lymphopoiesis. Absent TPO signaling in Mpl -/- mice led to enrichment of a common lymphoid progenitor (CLP) signature in multipotential lineage-negative Sca-1+c-Kit+ (LSK) cells and an increase in CLP formation. Moreover, Mpl -/- mice exhibited increased numbers of PreB2 and immature B-cells in bone marrow and spleen, with an increased proportion of B-lymphoid cells in the G1 phase of the cell cycle. Conversely, elevated TPO signaling in Tpo Tg mice was associated with reduced B-lymphopoiesis. Although at steady state, peripheral blood lymphocyte counts were normal in both models, Mpl -/- Eµ-myc mice showed an enhanced preneoplastic phase with increased numbers of splenic PreB2 and immature B-cells, a reduced quiescent fraction, and augmented blood lymphocyte counts. Thus, although Mpl is not expressed on lymphoid cells, TPO signaling may indirectly influence B-lymphopoiesis and the preneoplastic state in Myc-driven B-cell lymphomagenesis by lineage priming in multipotential progenitor cells.

Neutrophil macroaggregates promote widespread pulmonary thrombosis after gut ischemia.

Gut ischemia is common in critically ill patients, promoting thrombosis and inflammation in distant organs. The mechanisms linking hemodynamic changes in the gut to remote organ thrombosis remain ill-defined. We demonstrate that gut ischemia in the mouse induces a distinct pulmonary thrombotic disorder triggered by neutrophil macroaggregates. These neutrophil aggregates lead to widespread occlusion of pulmonary arteries, veins, and the microvasculature. A similar pulmonary neutrophil-rich thrombotic response occurred in humans with the acute respiratory distress syndrome. Intravital microscopy during gut ischemia-reperfusion injury revealed that rolling neutrophils extract large membrane fragments from remnant dying platelets in multiple organs. These platelet fragments bridge adjacent neutrophils to facilitate macroaggregation. Platelet-specific deletion of cyclophilin D, a mitochondrial regulator of cell necrosis, prevented neutrophil macroaggregation and pulmonary thrombosis. Our studies demonstrate the existence of a distinct pulmonary thrombotic disorder triggered by dying platelets and neutrophil macroaggregates. Therapeutic targeting of platelet death pathways may reduce pulmonary thrombosis in critically ill patients.

Undercover Agents: Targeting Tumours with Modified Platelets.

Platelets have long been recognised to colocalise with tumour cells throughout haematogenous metastasis. Interactions between these cells contribute to tumour cell survival and motility through the vasculature into other tissues. Now, the research focus is shifting towards developing means to exploit this relationship to provide accurate diagnostics and therapies. Alterations to platelet count, RNA profile, and platelet ultrastructure are associated with the presence of certain malignancies, and may be used for cancer detection. Additionally, nanoparticle-based drug delivery systems are enhanced through the use of platelet membranes to specifically target cancer cells and camouflage the foreign particles from the immune system. This review discusses the development of platelets into highly powerful tools for cancer diagnostics and therapies.

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.

The Pseudokinase MLKL and the Kinase RIPK3 Have Distinct Roles in Autoimmune Disease Caused by Loss of Death-Receptor-Induced Apoptosis.

The kinases RIPK1 and RIPK3 and the pseudo-kinase MLKL have been identified as key regulators of the necroptotic cell death pathway, although a role for MLKL within the whole animal has not yet been established. Here, we have shown that MLKL deficiency rescued the embryonic lethality caused by loss of Caspase-8 or FADD. Casp8(-/-)Mlkl(-/-) and Fadd(-/-)Mlkl(-/-) mice were viable and fertile but rapidly developed severe lymphadenopathy, systemic autoimmune disease, and thrombocytopenia. These morbidities occurred more rapidly and with increased severity in Casp8(-/-)Mlkl(-/-) and Fadd(-/-)Mlkl(-/-) mice compared to Casp8(-/-)Ripk3(-/-) or Fadd(-/-)Ripk3(-/-) mice, respectively. These results demonstrate that MLKL is an essential effector of aberrant necroptosis in embryos caused by loss of Caspase-8 or FADD. Furthermore, they suggest that RIPK3 and/or MLKL may exert functions independently of necroptosis. It appears that non-necroptotic functions of RIPK3 contribute to the lymphadenopathy, autoimmunity, and excess cytokine production that occur when FADD or Caspase-8-mediated apoptosis is abrogated.

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.