Escaping the nuclear confines: signal-dependent pre-mRNA splicing in anucleate platelets

Proteomics unravels platelet function

In this issue of Blood, Schulz and colleagues report promising findings using differential proteomics as a discovery tool to identify functionally important proteins in platelet activation. The authors identify several proteins that change in abundance upon platelet activation. These findings implicate several novel pathomechanisms relevant to platelet activation and identify novel potential therapeutic targets for platelet inhibition.1

Platelet tissue factor synthesis in type 2 diabetic patients is resistant to inhibition by insulin

OBJECTIVE

Patients with type 2 diabetes have an increased risk of cardiovascular disease and show abnormalities in the coagulation cascade. We investigated whether increased synthesis of tissue factor (TF) by platelets could contribute to the hypercoagulant state.

RESEARCH DESIGN AND METHODS

Platelets from type 2 diabetic patients and matched control subjects were adhered to different surface-coated proteins, and TF premRNA splicing, TF protein, and TF procoagulant activity were measured.

RESULTS

Different adhesive proteins induced different levels of TF synthesis. A mimetic of active clopidogrel metabolite (AR-C69931 MX) reduced TF synthesis by 56 +/- 10%, an aspirin-like inhibitor (indomethacin) by 82 +/- 9%, and the combination by 96 +/- 2%, indicating that ADP release and thromboxane A(2) production followed by activation of P2Y12 and thromboxane receptors mediate surface-induced TF synthesis. Interference with intracellular pathways revealed inhibition by agents that raise cAMP and interfere with phosphatidylinositol 3-kinase/protein kinase B. Insulin is known to raise cAMP in platelets and inhibited collagen III-induced TF premRNA splicing and reduced TF activity by 35 +/- 5 and 47 +/- 5% at 1 and 100 nmol/l. Inhibition by insulin was reduced in type 2 diabetes platelets resulting in an approximately 1.6-fold higher TF synthesis than in matched control subjects.

CONCLUSIONS

We characterized the extra- and intracellular mechanisms that couple surface activation to TF synthesis in adhering platelets. In healthy individuals, TF synthesis is inhibited by insulin, but in patients with type 2 diabetes inhibition is impaired. This leads to the novel finding that platelets from type 2 diabetic patients produce more TF than platelets from matched control subjects.

Evaluation of the antioxidant properties of N-acetylcysteine in human platelets: prerequisite for bioconversion to glutathione for antioxidant and antiplatelet activity

N-Acetylcysteine (NAC) is a frequently used "antioxidant" in vitro, but the concentrations applied rarely correlate with those encountered with oral dosing in vivo. Here, we investigated the in vitro antioxidant and antiplatelet properties of NAC at concentrations (10-100 microM) that are achievable in plasma with tolerable oral dosing. The impact of NAC pretreatment (2 hours) on aggregation of platelets from healthy volunteers in response to thrombin and adenosine diphosphate and on platelet-derived nitric oxide (NO) was examined. NAC was found to be a weak reducing agent and a poor antioxidant compared with glutathione (reduced form) (GSH). However, platelets treated with NAC showed enhanced antioxidant activity and depression of reactive oxygen species generation associated with increases in intraplatelet GSH levels. An approximately 2-fold increase in NO synthase-derived nitrite was observed with 10 microM NAC treatment, but the effect was not concentration dependent. Finally, NAC significantly reduced both thrombin-induced and adenosine diphosphate-induced platelet aggregation. NAC should be considered a weak antioxidant that requires prior conversion to GSH to convey antioxidant and antithrombotic benefit at therapeutically relevant concentrations. Our results suggest that NAC might be an effective antiplatelet agent in conditions where increased oxidative stress contributes to heightened risk of thrombosis but only if the intraplatelet machinery to convert it to GSH is functional.

De novo protein synthesis in mature platelets: a consideration for transfusion medicine

Platelet function in thrombosis and haemostasis is reasonably well understood at the molecular level with respect to the proteins involved in cellular structure, signalling networks and platelet interaction with clotting factors and other cells. However, the natural history of these proteins has only recently garnered the attention of platelet researchers. De novo protein synthesis in platelets was discovered 40 years ago; however, it was generally dismissed as merely an interesting minor phenomenon until studies over the past few years renewed interest in this aspect of platelet proteins. It is now accepted that anucleate platelets not only have the potential to synthesize proteins, but this capacity seems to be required to fulfil their function. With translational control as the primary mode of regulation, platelets are able to express biologically relevant gene products in a timely and signal-dependent manner. Platelet protein synthesis during storage of platelet concentrates is a nascent area of research. Protein synthesis does occur, although not for all proteins found in the platelet protein profile. Furthermore, mRNA appears to be well preserved under standard storage conditions. Although its significance is not yet understood, the ability to replace proteins may form a type of cellular repair mechanism during storage. Disruption by inappropriate storage conditions or processes that block protein synthesis such as pathogen reduction technologies may have direct effects on the ability of platelets to synthesize proteins during storage.

The intrinsic complement regulator decay-accelerating factor modulates the biological response to vascular injury

OBJECTIVE

To investigate whether the presence of decay-accelerating factor (or CD55), an intrinsic complement regulator, protects against the development of vascular disease, given that complement activation can affect leukocytes and platelets.

METHODS AND RESULTS

Leukocyte-platelet complexes are critical for the initiation and progression of atherosclerosis and restenosis; however, the mechanism by which these processes promote vascular injury is incompletely defined. We performed femoral artery wire injury in Daf1(-/-) mice and their wild-type controls. Leukocyte accumulation, cellular proliferation, and neointimal thickening were enhanced in Daf1(-/-) mice versus wild-type mice. Deficiency of either the C3a or the C5a receptor, respectively, reversed the increased vascular inflammation, cellular proliferation, and neointimal formation in Daf1(-/-) mice.

CONCLUSIONS

Decay-accelerating factor control of C3a and C5a generation and prevention of the binding of these activation fragments to the C3a and C5a receptors are critical for the biological response to vascular injury. Targeting the C3a and C5a receptors may be useful for the prevention of neointimal hyperplasia.

RUNX1/core binding factor A2 regulates platelet 12-lipoxygenase gene (ALOX12): studies in human RUNX1 haplodeficiency

Haploinsufficiency of RUNX1 (also known as CBFA2/AML1) is associated with familial thrombocytopenia, platelet dysfunction, and predisposition to acute leukemia. We have reported on a patient with thrombocytopenia and impaired agonist-induced aggregation, secretion, and protein phosphorylation associated with a RUNX1 mutation. Expression profiling of platelets revealed approximately 5-fold decreased expression of 12-lipoxygenase (12-LO, gene ALOX12), which catalyzes 12-hydroxyeicosatetraenoic acid production from arachidonic acid. We hypothesized that ALOX12 is a direct transcriptional target gene of RUNX1. In present studies, agonist-induced platelet 12-HETE production was decreased in the patient. Four RUNX1 consensus sites were identified in the 2-kb promoter region of ALOX12 (at -1498, -1491, -708, -526 from ATG). In luciferase reporter studies in human erythroleukemia cells, mutation of each site decreased activity; overexpression of RUNX1 up-regulated promoter activity, which was abolished by mutation of RUNX1 sites. Gel shift studies, including with recombinant protein, revealed RUNX1 binding to each site. Chromatin immunoprecipitation revealed in vivo RUNX1 binding in the region of interest. siRNA knockdown of RUNX1 decreased RUNX1 and 12-LO proteins. ALOX12 is a direct transcriptional target of RUNX1. Our studies provide further proof of principle that platelet expression profiling can elucidate novel alterations in platelets with inherited dysfunction.

VAMP8/endobrevin is overexpressed in hyperreactive human platelets: suggested role for platelet microRNA

BACKGROUND

Variation in platelet reactivity contributes to disorders of hemostasis and thrombosis, but the molecular mechanisms are not well understood.

OBJECTIVES

To discover associations between interindividual platelet variability and the responsible platelet genes, and to begin to define the molecular mechanisms altering platelet gene expression.

SUBJECTS/METHODS

Two hundred and eighty-eight healthy subjects were phenotyped for platelet responsiveness. Platelet RNA from subjects demonstrating hyperreactivity (n=18) and hyporeactivity (n=11) was used to screen the human transcriptome.

RESULTS

Distinctly different mRNA profiles were observed between subjects with differing platelet reactivity. Increased levels of mRNA for VAMP8/endobrevin, a critical v-SNARE involved in platelet granule secretion, were associated with platelet hyperreactivity (Q=0.0275). Validation studies of microarray results showed 4.8-fold higher mean VAMP8 mRNA levels in hyperreactive than hyporeactive platelets (P=0.0023). VAMP8 protein levels varied 13-fold among platelets from these normal subjects, and were 2.5-fold higher in hyperreactive platelets (P=0.05). Among our cohort of 288 subjects, a VAMP8 single-nucleotide polymorphism (rs1010) was associated with platelet reactivity in an age-dependent manner (P<0.003). MicroRNA-96 was predicted to bind to the 3'-untranslated regionof VAMP8 mRNA and was detected in platelets. Overexpression of microRNA-96 in VAMP8-expressing cell lines caused a dose-dependent decrease in VAMP8 protein and mRNA, suggesting a role in VAMP8 mRNA degradation.

CONCLUSIONS

These findings support a role for VAMP8/endobrevin in the heterogeneity of platelet reactivity, and suggest a role for microRNA-96 in the regulation of VAMP8 expression.

Anucleate platelets generate progeny

Platelets are classified as terminally differentiated cells that are incapable of cellular division. However, we observe that anucleate human platelets, either maintained in suspension culture or captured in microdrops, give rise to new cell bodies packed with respiring mitochondria and alpha-granules. Platelet progeny formation also occurs in whole blood cultures. Newly formed platelets are structurally indistinguishable from normal platelets, are able to adhere and spread on extracellular matrix, and display normal signal-dependent expression of surface P-selectin and annexin V. Platelet progeny formation is accompanied by increases in biomass, cellular protein levels, and protein synthesis in expanding populations. Platelet numbers also increase during ex vivo storage. These observations indicate that platelets have a previously unrecognized capacity for producing functional progeny, which involves a form of cell division that does not require a nucleus. Because this new function of platelets occurs outside of the bone marrow milieu, it raises the possibility that thrombopoiesis continues in the bloodstream.

Platelets and megakaryocytes contain functional nuclear factor-kappaB

OBJECTIVE

To investigate the presence and role of NF-kappaB proteins in megakaryocytes and platelets. The nuclear factor-kappaB (NF-kappaB) transcription factor family is well known for its role in eliciting inflammation and promoting cell survival. We discovered that human megakaryocytes and platelets express the majority of NF-kappaB family members, including the regulatory inhibitor-kappaB (I-kappaB) and I-kappa kinase (IKK) molecules.

METHODS AND RESULTS

Anucleate platelets exposed to NF-kappaB inhibitors demonstrated impaired fundamental functions involved in repairing vascular injury and thrombus formation. Specifically, NF-kappaB inhibition diminished lamellapodia formation, decreased clot retraction times, and reduced thrombus stability. Moreover, inhibition of I-kappaB-alpha phosphorylation (BAY-11-7082) reverted fully spread platelets back to a spheroid morphology. Addition of recombinant IKK-beta or I-kappaB-alpha protein to BAY inhibitor-treated platelets partially restored platelet spreading in I-kappaB-alpha inhibited platelets, and addition of active IKK-beta increased endogenous I-kappaB-alpha phosphorylation levels.

CONCLUSIONS

These novel findings support a crucial and nonclassical role for the NF-kappaB family in modulating platelet function and reveal that platelets are sensitive to NF-kappaB inhibitors. As NF-kappaB inhibitors are being developed as antiinflammatory and anticancer agents, they may have unintended effects on platelets. On the basis of these data, NF-kappaB is also identified as a new target to dampen unwanted platelet activation.

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

A fraction of fibrinogen contains a differently spliced γ chain called γ′, which presents itself mainly as heterodimer with the common γA chain as γA/γ′ fibrinogen. The γ′ chain differs from the γA chain in its C-terminus and has important functional implications for fibrinogen. The presence of the γ′ chain modulates thrombin and FXIII activity, influences clot architecture, and eliminates a platelet-binding site. Associations of γA/γ′ fibrinogen levels with arterial and venous thrombosis have been reported, indicating that the functional effects of γA/γ′ fibrinogen may contribute to the pathology of thrombosis. This review summarizes the key biologic aspects of this interesting variant of fibrinogen and discusses inconsistencies in current reports.

Class prediction models of thrombocytosis using genetic biomarkers

Criteria for distinguishing among etiologies of thrombocytosis are limited in their capacity to delineate clonal (essential thrombocythemia [ET]) from nonclonal (reactive thrombocytosis [RT]) etiologies. We studied platelet transcript profiles of 126 subjects (48 controls, 38 RT, 40 ET [24 contained the JAK2V(617)F mutation]) to identify transcript subsets that segregated phenotypes. Cross-platform consistency was validated using quantitative real-time polymerase chain reaction (RT-PCR). Class prediction algorithms were developed to assign phenotypic class between the thrombocytosis cohorts, and by JAK2 genotype. Sex differences were rare in normal and ET cohorts (< 1% of genes) but were male-skewed for approximately 3% of RT genes. An 11-biomarker gene subset using the microarray data discriminated among the 3 cohorts with 86.3% accuracy, with 93.6% accuracy in 2-way class prediction (ET vs RT). Subsequent quantitative RT-PCR analysis established that these biomarkers were 87.1% accurate in prospective classification of a new cohort. A 4-biomarker gene subset predicted JAK2 wild-type ET in more than 85% patient samples using either microarray or RT-PCR profiling, with lower predictive capacity in JAK2V(617)F mutant ET patients. These results establish that distinct genetic biomarker subsets can predict thrombocytosis class using routine phlebotomy.

"Message in the platelet"--more than just vestigial mRNA!

Although mammalian platelets are anucleated cells, a number of studies have shown that they retain a pool of messenger RNA (mRNA) carried over from the megakaryocyte during thrombopoiesis. Platelet mRNA was originally thought to be relatively unstable and short-lived within the youngest cells and has been used as a potential marker of platelet turnover. In this article we will discuss both theoretical and methodological issues related to the measurement of these younger, "reticulated platelets". A key question relating to platelet mRNA is also whether it has any functional relevance other than a marker of platelet immaturity. Evidence going back more than 30 years suggests that platelets can biosynthesize proteins. However, it is only very recently that the nature and specificity of platelet mRNA has been examined in any detail. Difficulties in obtaining pure platelet mRNA, free of contamination from other cells has added to the complexity of unravelling this story. However, there is now clear evidence that platelets contain small but significant levels of message for a variety of proteins. The platelet mRNA pool is much richer and more diverse than previously thought and recent data suggests that regulated synthesis of a selected number of proteins can be induced on platelet activation. The full complexity of the platelet genome is now just being revealed and may open the possibility for improved diagnosis and therapy of many haemostatic and thrombotic disorders.

Existence of a microRNA pathway in anucleate platelets

Platelets have a crucial role in the maintenance of hemostasis as well as in thrombosis and vessel occlusion, which underlie stroke and acute coronary syndromes. Anucleate platelets contain mRNAs and are capable of protein synthesis, raising the issue of how these mRNAs are regulated. Here we show that human platelets harbor an abundant and diverse array of microRNAs (miRNAs), which are known as key regulators of mRNA translation in other cell types. Further analyses revealed that platelets contain the Dicer and Argonaute 2 (Ago2) complexes, which function in the processing of exogenous miRNA precursors and the control of specific reporter transcripts, respectively. Detection of the receptor P2Y(12) mRNA in Ago2 immunoprecipitates suggests that P2Y(12) expression may be subjected to miRNA control in human platelets. Our study lends an additional level of complexity to the control of gene expression in these anucleate elements of the cardiovascular system.

Foxp3 regulates megakaryopoiesis and platelet function

OBJECTIVE

Platelets are crucial for hemostasis and are vital regulators of inflammation. Foxp3 is a key transcription factor for T regulatory cell development. Humans with IPEX (immune dysregulation, polyendocrinopathy, enteropathy, x-linked) and the scurfy (Foxp3(sf)) mouse have mutations in the Foxp3 gene that lead to a host of pathologies including autoimmunity and skin diseases. Scurfy mice and some humans with IPEX are also thrombocytopenic. The purpose of this study was to determine whether the absence of functional Foxp3 leads to defects in megakaryocytes and platelets.

METHODS AND RESULTS

We discovered that human and mouse megakaryocytes express Foxp3 mRNA and protein. Using shRNA and Foxp3(sf) mice, we demonstrated that Foxp3-deficient mouse and human megakaryocyte progenitors exhibited proliferation defects. Striking platelet abnormalities were observed in both an IPEX patient and Foxp3(sf) mice. Impaired platelet spreading and release of TGF-beta and CD40 ligand (CD40L), and abnormal levels of plasma CD40L were observed in a case of IPEX syndrome. Foxp3(sf) mice were thrombocytopenic and had increased platelet volume and altered serum levels of CD40L, TXB(2), and TGF-beta.

CONCLUSIONS

These findings provide compelling new evidence that Foxp3 is needed for proper megakaryopoiesis and plays a role in regulating platelet function including spreading and release.

Fibrinogen is required for maintenance of platelet intracellular and cell-surface P-selectin expression

Platelet P-selectin plays important roles in inflammation and contributes to thrombosis and hemostasis. Although it has been reported that von Willebrand factor (VWF) affects P-selectin expression on endothelial cells, little information is available regarding regulation of platelet P-selectin expression. Here, we first observed that P-selectin expression was significantly decreased on platelets of fibrinogen and VWF double-deficient mice. Subsequently, we identified this was due to fibrinogen deficiency. Impaired P-selectin expression on fibrinogen-deficient platelets was further confirmed in human hypofibrinogenemic patients. We demonstrated that this impairment is unlikely due to excessive P-selectin shedding, deficient fibrinogen-mediated cell surface P-selectin binding, or impaired platelet granule release, but rather is due to decreased platelet P-selectin content. Fibrinogen transfusion completely recovered this impairment in fibrinogen-deficient (Fg−/−) mice, and engagement of the C-terminus of the fibrinogen γ chain with β3 integrin was required for this process. Furthermore, Fg−/− platelets significantly increased P-selectin expression following transfusion into β3 integrin–deficient mice and when cultured with fibrinogen. These data suggest fibrinogen may play important roles in inflammation, thrombosis, and hemostasis via enhancement of platelet P-selectin expression. Since human fibrinogen levels vary significantly in normal and diseased populations, P-selectin as an activation marker on platelets should be used with caution.

Regulation of megakaryocyte maturation and platelet formation

Each day in every human, approximately 1 x 10(11) platelets are produced by the cytoplasmic fragmentation of megakaryocytes (MK), their marrow precursor cells. Platelets are the predominating factor in the process of hemostasis and thrombosis. Recent studies have shown that platelets also play a hitherto unsuspected role in several other processes such as inflammation, innate immunity, neoangiogenesis and tumor metastasis. The late phases of MK differentiation identified by polyploidization, maturation and organized fragmentation of the cytoplasm leading to the release of platelets in the blood stream represent a unique model of differentiation. The molecular and cellular mechanisms regulating platelet biogenesis are better understood and may explain several platelet disorders. This review focuses on MK polyploidization, and platelet formation, and discusses their alteration in some platelet disorders.

Platelet proteome changes associated with diabetes and during platelet storage for transfusion

Human platelets play a key role in hemostasis and thrombosis and have recently emerged as key regulators of inflammation. Platelets stored for transfusion produce pro-thrombotic and pro-inflammatory mediators implicated in adverse transfusion reactions. Correspondingly, these mediators are central players in pathological conditions including cardiovascular disease, the major cause of death in diabetics. In view of this, a mass spectrometry based proteomics study was performed on platelets collected from healthy and type-2 diabetics stored for transfusion. Strikingly, our innovative and sensitive proteomic approach identified 122 proteins that were either up- or down-regulated in type-2 diabetics relative to nondiabetic controls and 117 proteins whose abundances changed during a 5-day storage period. Notably, our studies are the first to characterize the proteome of platelets from diabetics before and after storage for transfusion. These identified differences allow us to formulate new hypotheses and experimentation to improve clinical outcomes by targeting "high risk platelets" that render platelet transfusion less effective or even unsafe.

[Platelet immunology and the immune response]

Platelets exert not only hemostatic activities, but also pro-inflammatory effects. Platelet-linked inflammation seems essentially related to their capacity of secreting cytokines, chemokines and related molecules. This activity is important in terms of concentration of secreted products. This secretory function confers to platelets a regulatory role in immunity. Besides, platelets do exhibit non-self infectious danger detection molecules on their surfaces, belonging in particular to the "Toll-like receptor family"; through this property, platelets can bind infectious agents but also deliver differential signals for the secretion of cytokines and chemokines. Platelets, which are non-nucleated cells deprived of nuclear DNA, possess however some cellular machinery which permits intracellular signalling and even the production of RNA transcripts for certain cytokines. Last, platelets express variant surface determinants of hemostatic molecules (referred to as HPA antigens) along with HLA class I variant molecules, the function of which on platelets is still unknown. An intriguing question is to reconcile those diverse properties and to understand whether the pro-inflammatory secretory process can affect the immunogenicity of transfused, allogeneic, platelet components.

Megakaryocyte impairment by eptifibatide-induced antibodies causes prolonged thrombocytopenia

Glycoprotein (GP) IIbIIIa inhibitors are used in the treatment of acute coronary syndromes. Transient immune-mediated acute thrombocytopenia is a recognized side effect of GPIIbIIIa inhibitors. We provide evidence that GPIIbIIIa inhibitor-induced antibodies can affect megakaryocytes in the presence of eptifibatide. In a patient with acute coronary syndrome, acute thrombocytopenia occurred after a second exposure to eptifibatide 20 days after the initial treatment. Despite the short half-life of eptifibatide (t(1/2) = 2 hours), thrombocytopenia less than 5 x 10(9)/L and gastrointestinal and skin hemorrhage persisted for 4 days. Glycoprotein-specific enzyme-linked immunosorbent assay showed eptifibatide-dependent, GPIIbIIIa-specific antibodies. Bone marrow examination showed predominance of early megakaryocyte stages, and platelet transfusion resulted in an abrupt platelet count increase. Viability of cultured cord blood-derived megakaryocytes was reduced in the presence of eptifibatide and patient IgG fraction. These findings can be explained by impaired megakaryocytopoiesis complicating anti-GPIIbIIIa antibody-mediated immune thrombocytopenia. This mechanism may also apply to some patients with autoimmune thrombocytopenia.

Tissue inhibitors of matrix metalloproteinases in platelets and megakaryocytes: a novel organization for these secreted proteins

OBJECTIVE

Expression of tissue inhibitors of matrix metalloproteinases (TIMPs) is one way that activated platelets intervene in tissue remodeling and angiogenesis. Our study was designed to investigate their synthesis in megakaryocytes (MKs) and their storage in platelets.

MATERIALS AND METHODS

TIMP expression in MKs derived from blood CD34(+) progenitor cells of normal donors and a megakaryocytic cell line (CHRF-288-11) grown in serum-free conditions and platelets from normal donors or two patients with gray platelet syndrome was studied by immunofluorescence labeling, reverse transcription-polymerase chain reaction, and western blotting.

RESULTS

Biosynthesis of TIMPs 1-4 in MKs was indicated by presence of their messenger RNAs as shown by polymerase chain reaction and of their proteins. Immunofluorescence labeling suggested a primarily granular localization of TIMPs in MKs and platelets. But when colocalization with von Willebrand factor, fibrinogen, P-selectin, and other alpha-granule proteins was assessed in platelets by confocal microscopy, TIMP-1, -2, and -4 were localized as distinct fluorescent patches apart from the established alpha-granule markers and largely independent of platelet metalloproteinases. TIMP-3 differed for it also had an alpha-granule location. Western blotting confirmed the presence of TIMPs 1-4 in platelets and thrombin activation resulted in their extensive release to the medium. Platelets from two patients with gray platelet syndrome, congenitally deficient in alpha-granules, showed sparse labeling of von Willebrand factor and fibrinogen confined to vestigial alpha-granules; however, localization of the TIMPs was unchanged.

CONCLUSIONS

TIMPs are synthesized and organized in MKs and platelets independently of other secreted proteins present in alpha-granule pools.

PAF-acetylhydrolase expressed during megakaryocyte differentiation inactivates PAF-like lipids

Platelet activating factor (PAF) and PAF-like lipids induce inflammatory responses in target cells. These lipid mediators are inactivated by PAF-acetylhydrolase (PAF-AH). The PAF signaling system affects the growth of hematopoietic CD34(+) cells, but roles for PAF-AH in this process are unknown. Here, we investigated PAF-AH function during megakaryopoiesis and found that human CD34(+) cells accumulate this enzymatic activity as they differentiate toward megakaryocytes, consistent with the expression of mRNA and protein for the plasma PAF-AH isoform. Inhibition of endogenous PAF-AH activity in differentiated megakaryocytes increased formation of lipid mediators that signaled the PAF receptor (PAFR) in fully differentiated human cells such as neutrophils, as well as megakaryocytes themselves. PAF-AH also controlled megakaryocyte alpha(IIb)beta(3)-dependent adhesion, cell spreading, and mobility that relied on signaling through the PAFR. Together these data suggest that megakaryocytes generate PAF-AH to modulate the accumulation of intracellular phospholipid mediators that may detrimentally affect megakaryocyte development and function.

Dominant inheritance of a novel integrin beta3 mutation associated with a hereditary macrothrombocytopenia and platelet dysfunction in two Italian families

BACKGROUND

Defects of integrin alpha(IIb)beta(3) are typical of Glanzmann's thrombasthenia, an inherited autosomal recessive bleeding disorder characterized by the failure of platelets to aggregate in response to all physiological agonists, but with no abnormalities in the number or size of platelets. Although large heterogeneity has been described for Glanzmann's thrombasthenia, no family has so far been described as having an autosomal dominant form of this disease.

DESIGN AND METHODS

We describe two Italian families with moderate thrombocytopenia with large platelets, defective platelet function and moderate/severe mucocutaneous bleeding, transmitted as an autosomal dominant trait and associated with a novel integrin beta(3)-gene (ITGB3) mutation.

RESULTS

The characteristics of our families are moderate macrothrombocytopenia and defective platelet function associated with a mild reduction of surface alpha(Ib) beta(3), impaired platelet aggregation to physiological agonists but not to ristocetin, normal clot retraction, reduced fibrinogen binding and expression of activated alpha(IIb)beta(3) upon stimulation, normal platelet adhesion to immobilized fibrinogen but reduced platelet spreading and tyrosine phosphorylation, indicating defective alpha(IIb)beta(3)-mediated outside-in signaling. Molecular analysis revealed a novel mutation of ITGB3 that determines an in-frame deletion producing the loss of amino acids 647-686 of the betaTD ectodomain of integrin beta(3). Haplotype analysis indicated that the two families inherited the mutation from a common ancestral chromosome.

CONCLUSIONS

This novel autosomal dominant macrothrombocytopenia associated with platelet dysfunction raises interesting questions about the role of integrin beta(3), and its betaTD domain, in platelet formation and function.

High-throughput proteomics detection of novel splice isoforms in human platelets

Alternative splicing (AS) is an intrinsic regulatory mechanism of all metazoans. Recent findings suggest that 100% of multiexonic human genes give rise to splice isoforms. AS can be specific to tissue type, environment or developmentally regulated. Splice variants have also been implicated in various diseases including cancer. Detection of these variants will enhance our understanding of the complexity of the human genome and provide disease-specific and prognostic biomarkers. We adopted a proteomics approach to identify exon skip events - the most common form of AS. We constructed a database harboring the peptide sequences derived from all hypothetical exon skip junctions in the human genome. Searching tandem mass spectrometry (MS/MS) data against the database allows the detection of exon skip events, directly at the protein level. Here we describe the application of this approach to human platelets, including the mRNA-based verification of novel splice isoforms of ITGA2, NPEPPS and FH. This methodology is applicable to all new or existing MS/MS datasets.

Human platelets express organic anion-transporting peptide 2B1, an uptake transporter for atorvastatin

Statins are widely used to treat dyslipidemia. Effects of statins in addition to low-density lipoprotein lowering include altered platelet aggregation, requiring drug uptake into platelets. Possible candidates for mediating intraplatelet accumulation of statins include members of the organic anion-transporting polypeptide family such as OATP2B1 (SLCO2B1), a high-affinity uptake transporter for atorvastatin. Therefore, we analyzed OATP expression, localization, and function in human platelets. OATP2B1, but not OATP1B1, was detected in platelets and megakaryocytes on transcript and protein levels. Protein localization was almost exclusively confined to the plasma membrane. Moreover, we could demonstrate significant inhibition of estrone sulfate uptake into platelets by atorvastatin as well as direct transport of atorvastatin into platelets using a liquid chromatography-tandem mass spectrometry method. As a consequence of OATP2B1-mediated uptake of atorvastatin, we observed significant atorvastatin-mediated reduction of thrombin-induced Ca(2+) mobilization in platelets (37.3 +/- 6.7% of control at 15 microM atorvastatin), mechanistically explainable by reduced lipid modification of signal proteins. This effect was reversed by addition of mevalonate. Finally, we demonstrated expression of HMG-CoA reductase, the primary target of atorvastatin, in platelet cytosol. In conclusion, OATP2B1 is an uptake transporter expressed in platelets and is involved in statin-mediated alteration of platelet aggregation.

Protein synthesis by platelets: historical and new perspectives

In the late 1960s, numerous investigators independently demonstrated that platelets are capable of synthesizing proteins. Studies continued at a steady pace over the next 30 years and into the 21st century. Collectively, these investigations confirmed that platelets synthesize proteins and that the pattern of protein synthesis changes in response to cellular activation. More recent studies have characterized the mechanisms by which platelets synthesize proteins and have shown that protein synthesis alters the phenotype and functions of platelets. Here, we chronologically review our increased understanding of protein synthetic responses in platelets and discuss how the field may evolve over the next decade.

Transcript profiling of human platelets using microarray and serial analysis of gene expression (SAGE)

Platelets are anucleated cells that are generated from megakaryocytes via thrombopoiesis. They lack genomic DNA but have a pool of individual mRNA transcripts. Taken together, these mRNAs constitute a platelet transcriptome. Platelets have a unique and reproducible transcript profile, which includes approximately 1,600-3,000 individual transcripts. In this chapter, we will focus on platelet purification and on transcript profiling using an Affymetrix microarray platform and serial analysis of gene expression (SAGE). Platelet purification is described in detail. Large-scale platelet purification schema is designed to purify platelets from apheresis platelet bags (approximately 3-5 x 10(11) platelets/bag). Modification of this schema --small-scale platelet purification--is designed to isolate platelets from 20 ml of peripheral blood. This chapter provides detailed protocols for microarray and SAGE transcript profiling. We also discuss peculiarities of platelet purification, RNA isolation, and transcript profiling.

Gene expression analysis of human red blood cells

Understanding of molecular mechanisms governing the enucleating phenomena of human erythrocytes is of major importance in both fundamental and applied studies. Total RNA (n=7) from human RBCs (purity of erythrocyte preparation >99,99%) was tested using 2100 Bioanalyzer (Agilent, USA), and transcribed to cDNA. Microarray analysis was performed with the Human Genome Focus GeneChip (Affymetrix, USA), containing 8500 transcripts corresponding to 8400 human genes. Here we report that human RBCs contain typical eukaryotic RNA with 28S- and18S-rRNA standard bands. Microarray studies revealed the presence of transcripts of 1019 different genes in erythrocytic RNA. Gene Ontology analysis recognized 859 genes involved in general biological processes: 529 genes for cellular metabolism, 228 genes for signal transduction, 104 genes for development, 107 genes for immune response, 62 genes for protein localization, 53 genes for programmed cell death, and 5 genes for autophagy. A number of genes responsible for transcription, translation, RNA-stabilisation as well as for apoptosis and anti-apoptosis have been identified for the first time in circulating human RBCs. The presented data shed new light on the genetic determination of erythropoiesis, apoptosis and may have implications on the pathophysiology and diagnosis of various diseases involving red blood cells.

Platelets influence vascularized organ transplants from start to finish

This review relates the basic functions of platelets to specific aspects of organ allograft rejection. Platelet activation can occur in the donor or recipient before transplantation as well as during antibody- and cell-mediated rejection. Biopsies taken during organ procurement from cadaver donors have documented that activated platelets are attached to vascular endothelial cells or leukocytes. In addition, many patients waiting for transplants have activated platelets due to the diseases that lead to organ failure or as a result of interventions used to support patients before and during transplantation. The contribution of platelets to hyperacute rejection of both allografts and xenografts is well recognized. Intravascular aggregates of platelets can also be prominent in experimental and clinical transplants that undergo acute antibody or cell-mediated rejection. In acute rejection, platelets can recruit mononuclear cells by secretion of chemokines. After contact, monocytes, macrophages and T cells interact with platelets through receptor/ligand pairs, including P-selectin/PSGL-1 and CD40/CD154. There is a potential for therapy to inhibit platelet mediated immune stimulation, but it is counterbalanced by the need to maintain coagulation in the perioperative period.

Isolation and characterization of post-splicing lariat-intron complexes

Pre-mRNA splicing occurs in a large complex spliceosome. The steps of both spliceosome assembly and splicing reaction have been extensively analyzed, and many of the factors involved have been identified. However, the post-splicing intron turnover process, especially in vertebrates, remains to be examined. In this paper, we developed a two-tag affinity purification method for purifying lariat intron RNA-protein complexes obtained from an in vitro splicing reaction. Glycerol gradient sedimentation analyses revealed that there are at least two forms of post-splicing intron complexes, which we named the 'Intron Large (IL)' and the 'Intron Small (IS)' complexes. The IL complex contains U2, U5 and U6 snRNAs and other protein splicing factors, whereas the IS complex contains no such U snRNAs or proteins. We also showed that TFIP11, a human homolog of yeast Ntr1, is present in the IL complex and the TFIP11 mutant protein, which lacks the interaction domain with hPrp43 protein, caused accumulation of the IL complex and reduction of IS complex formation in vitro. Taken together, our results strongly suggest that TFIP11 in cooperation with hPrp43 mediates the transition from the IL complex to the IS complex, leading to efficient debranching and turnover of excised introns.

Tissue factor storage, synthesis and function in normal and activated human platelets

The source and significance of blood-borne tissue factor (TF) are controversial. The presence of TF in platelets was initially attributed to transfer of the protein from other cells (e.g., monocytes) and/or TF-bearing microparticles. Recently, TF-mRNA, neo-synthesis of the protein and TF-dependent procoagulant activity (PCA) have been reported in human platelets. The storage of "encrypted", potentially active TF in circulating, non-stimulated platelets remains debatable. One report strongly suggests that the starting of platelet PCA depends on de novo TF synthesis induced by platelet activation, whereas others provide persuasive evidence that platelets circulate with preformed TF, readily functional upon demand. These findings may have an impact on our current ideas of physiological hemostasis and thrombus formation. In fact, platelets would lead not only the formation of the primary plug, but in this microenvironment they would also contribute to the triggering of thrombin generation, fibrin deposition, clot consolidation and initial protection from fibrinolysis. Much research is needed to validate this platelet-based hemostasis model.

The role of platelet activation in tumor metastasis

Platelets are highly reactive components of the circulatory system, which exert not only haemostatic activity but also contribute to the modulation of various pathological conditions including inflammation, atherosclerosis and cancer metastasis through the release of cytokines, chemokines and the presentation of several adhesion molecules. During cancer metastasis, the formation of platelet-tumor cell aggregates in the circulation facilitates immune evasion and the microvascular arrest of tumor cells at distant sites. Several adhesion molecules, such as integrins and glycoproteins, were shown to be involved in this process. Recent findings indicate that P-selectin is another main mediator of platelet-tumor cell interactions. Other effects of activated platelets on cancer progression are associated with a release of platelet-derived factors stimulating tumor growth and angiogenesis. Any interference in platelet-tumor cell interactions resulted in attenuation of cancer metastasis. The well recognized, albeit not fully characterized function of platelets during cancer progression defines platelets as potential targets for cancer therapy. Specifically, the rapid expression of P-selectin on the cell surface of activated platelets and its strong association with metastasis provide a rationale for P-selectin inhibition as an antimetastatic treatment.

Platelet factor 4 mediates inflammation in experimental cerebral malaria

Cerebral malaria (CM) is a major complication of Plasmodium falciparum infection in children. The pathogenesis of CM involves vascular inflammation, immune stimulation, and obstruction of cerebral capillaries. Platelets have a prominent role in both immune responses and vascular obstruction. We now demonstrate that the platelet-derived chemokine, platelet factor 4 (PF4)/CXCL4, promotes the development of experimental cerebral malaria (ECM). Plasmodium-infected red blood cells (RBCs) activated platelets independently of vascular effects, resulting in increased plasma PF4. PF4 or chemokine receptor CXCR3 null mice had less severe ECM, including decreased T cell recruitment to the brain, and platelet depletion or aspirin treatment reduced the development of ECM. We conclude that Plasmodium-infected RBCs can directly activate platelets, and platelet-derived PF4 then contributes to immune activation and T cell trafficking as part of the pathogenesis of ECM.

Lipopolysaccharide is a direct agonist for platelet RNA splicing

Platelets express TLR4 receptors, but its ligand LPS does not directly activate thrombotic functions nor, obviously, transcription by these anucleate cells. Platelets, however, store information that changes their phenotype over a few hours in the form of unprocessed RNA transcripts. We show even low concentrations of LPS in the presence of soluble CD14 initiated splicing of unprocessed IL-1beta RNA, with translation and accumulation of IL-1beta protein. LPS was a more robust agonist for this response than thrombin. Platelets also contained cyclooxygenase-2 pre-mRNA, which also was spliced and translated after LPS stimulation. Flow cytometry and immunocytochemistry of platelets extensively purified by negative immunodepletion showed platelets contained IL-1beta, and quantitative assessment of white blood cell contamination by CD14 real time PCR confirms that leukocytes were not the IL-1beta source, nor were they required for platelet stimulation. LPS did not initiate rapid platelet responses, but over time did prime platelet aggregation to soluble agonists, induced actin rearrangement, and initiated granule secretion with P-selectin expression that resulted the coating of quiescent leukocytes with activated platelets. LPS is a direct agonist for platelets that allows these cells to directly participate in the innate immune response to bacteria.

Functional expression of IgA receptor FcalphaRI on human platelets

FcalphaRI (CD89) is a human IgA FcR expressed on cells of myeloid lineage such as neutrophils, monocytes, tissue macrophages, eosinophils, and subpopulations of dendritic cells. FcalphaRI mediates cell activation through Src family kinases and downstream tyrosine-based phosphorylation pathways. However, the role of IgA and the expression and role of its cognate receptor FcalphaRI (CD89) in platelet activation are undefined. In the current study, we demonstrate that human platelets express FcalphaRI mRNAs and proteins. Furthermore, we show that the platelet FcalphaRI is associated with the FcR gamma-chain, and cross-linking of FcalphaRI leads to Syk phosphorylation. Clustering of FcalphaRI induces pre-mRNA splicing and protein production of tissue factor and IL-1beta, suggesting novel roles for human platelet FcalphaRI and serum IgA in thrombosis and inflammation.

Amicus or adversary: platelets in lung biology, acute injury, and inflammation

Platelets are the chief effector cells in hemostasis and have additional major functions in inflammation, vascular integrity, and tissue repair. Platelets and the lungs have interrelated activities. Previous studies provide evidence that platelets contribute to pulmonary vascular barrier function and are required for defense against pulmonary hemorrhage, and that the lungs can influence platelet number and distribution. There is also evidence that platelets contribute to pathologic syndromes of pulmonary inflammation and thrombosis. Thus, platelets have an "amicus or adversary" relationship with the lung. Recent observations and discoveries have established new paradigms relevant to influences of platelets on lung cell and molecular biology. These new findings are in a variety of areas including thrombopoieis, nontraditional activities of platelets, new synthetic capabilities and mechanisms of post-translational gene expression, interactions of platelets with endothelial cells and contributions to alveolar capillary barrier permeability, interactions of platelets with myeloid leukocytes, and platelet involvement in stem cell signaling and vascular repair. These issues are considered in a translational approach, with an emphasis on acute lung injury and the acute respiratory distress syndrome.

Platelet tissue factor: how did it get there and is it important?

Recently, the presence of functionally active tissue factor (TF) in platelets has been reported by several groups. In this location, TF is postulated to play an important role in the propagation phase of thrombus formation. Although the existence of platelet TF still remains controversial to some extent, a review of the current literature proposes at least three distinct sources of "platelet-associated TF" in those laboratories that have reported its presence: (1) TF that is taken up in the form of circulating microparticles, usually derived from monocytes; (2) TF stored in the alpha-granules of platelets that may have been taken up and/or endogenously synthesized; and (3) TF that is synthesized and expressed on the plasma membrane of mature platelets. These pathways are not mutually exclusive, and the dominant mechanism may depend on the state of platelet activation and, possibly, on other host factors that differ in physiological hemostasis versus pathological thrombosis. This brief review will summarize the state-of-the-art understanding on the origins and possible role of platelet TF.

Minor spliceosome components are predominantly localized in the nucleus

Recently, it has been reported that there is a differential subcellular distribution of components of the minor U12-dependent and major U2-dependent spliceosome, and further that the minor spliceosome functions in the cytoplasm. To study the subcellular localization of the snRNA components of both the major and minor spliceosomes, we performed in situ hybridizations with mouse tissues and human cells. In both cases, all spliceosomal snRNAs were nearly exclusively detected in the nucleus, and the minor U11 and U12 snRNAs were further shown to colocalize with U4 and U2, respectively, in human cells. Additionally, we examined the distribution of several spliceosomal snRNAs and proteins in nuclear and cytoplasmic fractions isolated from human cells. These studies revealed an identical subcellular distribution of components of both the U12- and U2-dependent spliceosomes. Thus, our data, combined with several earlier publications, establish that, like the major spliceosome, components of the U12-dependent spliceosome are localized predominantly in the nucleus.

Biomarkers and Bioassays for Cardiovascular Diseases: Present and Future

Stratification of cardiac patients arriving at the emergency department is now being made according to the levels of acute cardiac biomarkers (i.e. cardiac troponin (cTn) or creatine kinase myocardial band (CK-MB)). Ongoing efforts are undertaken in an attempt to identify and validate additional cardiac biomarkers, for example, interleukin-6, soluble CD40L, and C-reactive protein, in order to further risk stratify patients with acute coronary syndrome. Several studies have also now shown an association of platelet transcriptome and genomic single nucleotide polymorphisms with myocardial infarction by using advanced genomic tools. A number of markers, such as myeloid-related protein 14 (MRP-14), cyclooxygenase-1 (COX-1), 5-lipoxygenase activating protein (FLAP), leukotriene A4 hydrolase (LTA4H) and myocyte enhancing factor 2A (MEF2A), have been linked to acute coronary syndromes, including myocardial infarction. In the future, these novel markers may pave the way toward personalized disease-prevention programs based on a person's genomic, thrombotic and cardiovascular profiles. Current and future biomarkers and bioassays for identifying at-risk patients will be discussed in this review.

Biomarkers and Bioassays for Cardiovascular Diseases: Present and Future

Stratification of cardiac patients arriving at the emergency department is now being made according to the levels of acute cardiac biomarkers (i.e. cardiac troponin (cTn) or creatine kinase myocardial band (CK-MB)). Ongoing efforts are undertaken in an attempt to identify and validate additional cardiac biomarkers, for example, interleukin-6, soluble CD40L, and C-reactive protein, in order to further risk stratify patients with acute coronary syndrome. Several studies have also now shown an association of platelet transcriptome and genomic single nucleotide polymorphisms with myocardial infarction by using advanced genomic tools. A number of markers, such as myeloid-related protein 14 (MRP-14), cyclooxygenase-1 (COX-1), 5-lipoxygenase activating protein (FLAP), leukotriene A(4) hydrolase (LTA4H) and myocyte enhancing factor 2A (MEF2A), have been linked to acute coronary syndromes, including myocardial infarction. In the future, these novel markers may pave the way toward personalized disease-prevention programs based on a person's genomic, thrombotic and cardiovascular profiles. Current and future biomarkers and bioassays for identifying at-risk patients will be discussed in this review.

Signal-dependent protein synthesis by activated platelets: new pathways to altered phenotype and function

New biologic activities of platelets continue to be discovered, indicating that concepts of platelet function in hemostasis, thrombosis, and inflammation require reconsideration as new paradigms evolve. Studies done over 3 decades ago demonstrated that mature circulating platelets have protein synthetic capacity, but it was thought to be low level and inconsequential. In contrast, recent discoveries demonstrate that platelets synthesize protein products with important biologic activities in a rapid and sustained fashion in response to cellular activation. This process, termed signal-dependent translation, uses a constitutive transcriptome and specialized pathways, and can alter platelet phenotype and functions in a fashion that can have clinical relevance. Signal-dependent translation and consequent protein synthesis are examples of a diverse group of posttranscriptural mechanisms in activated platelets that are now being revealed.

Splicing segregation: the minor spliceosome acts outside the nucleus and controls cell proliferation

The functional relevance and the evolution of two parallel mRNA splicing systems in eukaryotes--a major and minor spliceosome that differ in abundance and splicing rate--are poorly understood. We report here that partially spliced pre-mRNAs containing minor-class introns undergo nuclear export and that minor-class snRNAs are predominantly cytoplasmic in vertebrates. Cytoplasmic interference with the minor spliceosome further indicated its functional segregation from the nucleus. In keeping with this, minor splicing was only weakly affected during mitosis. By selectively interfering with snRNA function in zebrafish development and in mammalian cells, we revealed a conserved role for minor splicing in cell-cycle progression. We argue that the segregation of the splicing systems allows for processing of partially unspliced cytoplasmic transcripts, emerging as a result of different splicing rates. The segregation offers a mechanism accounting for spliceosome evolution in a single lineage and provides a means for nucleus-independent control of gene expression.

Division of labor: minor splicing in the cytoplasm

There are two molecular machineries for pre-mRNA splicing-the major spliceosome and the minor spliceosome. In this issue of Cell, König et al. (2007) demonstrate that the two splicing pathways are spatially separated in the cell and may have distinct functions.