Autophagy is induced upon platelet activation and is essential for hemostasis and thrombosis

PI3K inhibitors in thrombosis and cardiovascular disease

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases that regulate important intracellular signalling and vesicle trafficking events via the generation of 3-phosphoinositides. Comprising eight core isoforms across three classes, the PI3K family displays broad expression and function throughout mammalian tissues, and the (patho)physiological roles of these enzymes in the cardiovascular system present the PI3Ks as potential therapeutic targets in settings such as thrombosis, atherosclerosis and heart failure. This review will discuss the PI3K enzymes and their roles in cardiovascular physiology and disease, with a particular focus on platelet function and thrombosis. The current progress and future potential of targeting the PI3K enzymes for therapeutic benefit in cardiovascular disease will be considered, while the challenges of developing drugs against these master cellular regulators will be discussed.

The Effect of Autophagic Activity on the Function of Apheresis Platelets and on the Efficacy of Clinical Platelet Transfusion

Platelet activation and survival jointly determine the efficacy of clinical platelet transfusion. This study aimed to discuss the effect of autophagic activity on activation and aggregation of apheresis platelets and on the efficacy of clinical platelet transfusion. In this study, we investigated the effects of autophagic activity of apheresis platelets for different blood types and after different storage durations on platelet activation and aggregation functions. By Western blot, immunofluorescence, and RT-qPCR detection, we found that with the prolongation of the storage duration, the expressions of both autophagy-related proteins and genes were upregulated in apheresis platelets and their expressions were insignificantly higher in the apheresis platelets of type A and O blood than in those of type B and type AB blood. After RAPA/IGF-1 pretreatment, there was a significant increase/reduction in autophagic activity. After RAPA and IGF-1 pretreatment, an opposite variation trend was observed with platelet activation and aggregation. Autophagic activity of platelets correlated negatively with the efficacy of clinical platelet transfusion. These research findings provide a theoretical basis for effective clinical platelet transfusion.

Internalization of microparticles by platelets is partially mediated by toll-like receptor 4 and enhances platelet thrombogenicity

BACKGROUND AND AIMS

Circulating platelet microparticles (PMP) are the most abundant in bloodstream, are highly procoagulant and contribute to cross-talk with inflammatory cells. The aim of the present study was to investigate the interactions of PMP with platelets and explore the involvement of toll-like receptor 4 (TLR-4).

METHODS

PMP were separated by ultracentrifugation of expired platelet concentrates and added to: i) washed platelets, to confirm uptake, by flow cytometry and confocal and transmission electron microscopy, ii) platelet rich plasma (PRP), to assess changes in platelet function due to uptake by aggregometry in response to ADP; and iii) whole blood, to evaluate heterotypic aggregate (HA) formation by flow cytometry. Moreover, whole blood previously enriched with platelets with internalized PMP was used to explore modifications in thromboelastometry parameters (ROTEM). The inhibitory action of anti-TLR-4 was investigated.

RESULTS

Confocal and ultrastructural microscopy studies revealed PMP internalization by platelets. Flow cytometry showed PMP-platelet association (p < 0.01 vs controls, at different PMP dilutions). PMP, at 1/20 dilution, increased HA (p < 0.05 vs controls), the percentage of maximal platelet aggregation to ADP (p < 0.05 vs controls), and accelerated clotting and clot formation times (p < 0.05 vs controls). Incubation of platelets with anti-TLR-4 prior to exposure to PMP reduced PMP-platelet association (p < 0.05 vs absence of the antibody), prevented HA formation, reduced maximal platelet aggregation and normalized ROTEM parameters.

CONCLUSIONS

Platelets exhibit internalization ability towards their own PMP, a process that potentiates their thrombogenicity and is partially mediated by the innate immunity receptor TLR-4.

Role of Platelet Mitochondria: Life in a Nucleus-Free Zone

Platelets are abundant, small, anucleate circulating cells, serving many emerging pathophysiological roles beyond hemostasis; including active critical roles in thrombosis, injury response, and immunoregulation. In the absence of genomic DNA transcriptional regulation (no nucleus), platelets require strategic prepackaging of all the needed RNA and organelles from megakaryocytes, to sense stress (e.g., hyperglycemia), to protect themselves from stress (e.g., mitophagy), and to communicate a stress response to other cells (e.g., granule and microparticle release). Distinct from avian thrombocytes that have a nucleus, the absence of a nucleus allows the mammalian platelet to maintain its small size, permits morphological flexibility, and may improve speed and efficiency of protein expression in response to stress. In the absence of a nucleus, platelet lifespan of 7–10 days, is largely determined by the mitochondria. The packaging of 5–8 mitochondria is critical in aerobic respiration and yielding metabolic substrates needed for function and survival. Mitochondria damage or dysfunction, as observed with several disease processes, results in greatly attenuated platelet survival and increased risk for thrombovascular events. Here we provide insights into the emerging roles of platelets despite the lack of a nucleus, and the key role played by mitochondria in platelet function and survival both in health and disease.

Phosphatidylinositol 3 monophosphate metabolizing enzymes in blood platelet production and in thrombosis

Blood platelets, produced by the fragmentation of megakaryocytes, play a key role in hemostasis and thrombosis. Being implicated in atherothrombosis and other thromboembolic disorders, they represent a major therapeutic target for antithrombotic drug development. Several recent studies have highlighted an important role for the lipid phosphatidylinositol 3 monophosphate (PtdIns3P) in megakaryocytes and platelets. PtdIns3P, present in small amounts in mammalian cells, is involved in the control of endocytic trafficking and autophagy. Its metabolism is finely regulated by specific kinases and phosphatases. Class II (α, β and γ) and III (Vps34) phosphoinositide-3-kinases (PI3Ks), INPP4 and Fig4 are involved in the production of PtdIns3P whereas PIKFyve, myotubularins (MTMs) and type II PIPK metabolize PtdIns3P. By regulating the turnover of different pools of PtdIns3P, class II (PI3KC2α) and class III (Vps34) PI3Ks have been recently involved in the regulation of platelet production and functions. These pools of PtdIns3P appear to modulate membrane organization and intracellular trafficking. Moreover, PIKFyve and INPP4 have been recently implicated in arterial thrombosis. In this review, we will discuss the role of PtdIns3P metabolizing enzymes in platelet production and function. Potential new anti-thrombotic therapeutic perspectives based on inhibitors targeting specifically PtdIns3P metabolizing enzymes will also be commented.

Autophagic regulation of platelet biology

Platelets, developed from megakaryocytes, are characterized by anucleate and short-life span hemocyte in mammal vessel. Platelets are very important in the cardiovascular system. Studies indicate the occurrence of autophagy platelets and megakaryocytes. Moreover, abnormal autophagy decreases the number of platelets and suppresses platelet aggregation. In addition, mitophagy, as a kind of selective autophagy, could inhibit platelet aggregation under oxidative stress or hypoxic, whereas promote platelet aggregation after reperfusion. Finally, autophagy regulates hemorrhagic and thrombosis diseases by influencing the number and function of platelets. In this paper, the role of autophagy in platelets and megakaryocytes, as well as coupled with the promotive or inhibitory role of hemorrhagic and thrombosis diseases are elucidated. Therefore, autophagy may be a potentially therapeutic target in modulating the platelet-related diseases.

Nix-mediated mitophagy regulates platelet activation and life span

Platelet activation requires fully functional mitochondria, which provide a vital energy source and control the life span of platelets. Previous reports have shown that both general autophagy and selective mitophagy are critical for platelet function. However, the underlying mechanisms remain incompletely understood. Here, we show that Nix, a previously characterized mitophagy receptor that plays a role in red blood cell maturation, also mediates mitophagy in platelets. Genetic ablation of Nix impairs mitochondrial quality, platelet activation, and FeCl3-induced carotid arterial thrombosis without affecting the expression of platelet glycoproteins (GPs) such as GPIb, GPVI, and αIIbβ3 Metabolic analysis revealed decreased mitochondrial membrane potential, enhanced mitochondrial reactive oxygen species level, diminished oxygen consumption rate, and compromised adenosine triphosphate production in Nix -/- platelets. Transplantation of wild-type (WT) bone marrow cells or transfusion of WT platelets into Nix-deficient mice rescued defects in platelet function and thrombosis, suggesting a platelet-autonomous role (acting on platelets, but not other cells) of Nix in platelet activation. Interestingly, loss of Nix increases the life span of platelets in vivo, likely through preventing autophagic degradation of the mitochondrial protein Bcl-xL. Collectively, our findings reveal a novel mechanistic link between Nix-mediated mitophagy, platelet life span, and platelet physiopathology. Our work suggests that targeting platelet mitophagy Nix might provide new antithrombotic strategies.

Mitochondrial MsrB2 serves as a switch and transducer for mitophagy

Mitophagy can selectively remove damaged toxic mitochondria, protecting a cell from apoptosis. The molecular spatial-temporal mechanisms governing autophagosomal selection of reactive oxygen species (ROS)-damaged mitochondria, particularly in a platelet (no genomic DNA for transcriptional regulation), remain unclear. We now report that the mitochondrial matrix protein MsrB2 plays an important role in switching on mitophagy by reducing Parkin methionine oxidation (MetO), and transducing mitophagy through ubiquitination by Parkin and interacting with LC3. This biochemical signaling only occurs at damaged mitochondria where MsrB2 is released from the mitochondrial matrix. MsrB2 platelet-specific knockout and in vivo peptide inhibition of the MsrB2/LC3 interaction lead to reduced mitophagy and increased platelet apoptosis. Pathophysiological importance is highlighted in human subjects, where increased MsrB2 expression in diabetes mellitus leads to increased platelet mitophagy, and in platelets from Parkinson's disease patients, where reduced MsrB2 expression is associated with reduced mitophagy. Moreover, Parkin mutations at Met192 are associated with Parkinson's disease, highlighting the structural sensitivity at the Met192 position. Release of the enzyme MsrB2 from damaged mitochondria, initiating autophagosome formation, represents a novel regulatory mechanism for oxidative stress-induced mitophagy.

Increased levels of the megakaryocyte and platelet expressed cysteine proteases stefin A and cystatin A prevent thrombosis

Increased platelet activity occurs in type 2 diabetes mellitus (T2DM) and such platelet dysregulation likely originates from altered megakaryopoiesis. We initiated identification of dysregulated pathways in megakaryocytes in the setting of T2DM. We evaluated through transcriptomic analysis, differential gene expressions in megakaryocytes from leptin receptor-deficient mice (db/db), exhibiting features of human T2DM, and control mice (db/+). Functional gene analysis revealed an upregulation of transcripts related to calcium signaling, coagulation cascade and platelet receptors in diabetic mouse megakaryocytes. We also evidenced an upregulation (7- to 9.7-fold) of genes encoding stefin A (StfA), the human ortholog of Cystatin A (CSTA), inhibitor of cathepsin B, H and L. StfA/CSTA was present in megakaryocytes and platelets and its expression increased during obesity and diabetes in rats and humans. StfA/CSTA was primarily localized at platelet membranes and granules and was released upon agonist stimulation and clot formation through a metalloprotease-dependent mechanism. StfA/CSTA did not affect platelet aggregation, but reduced platelet accumulation on immobilized collagen from flowing whole blood (1200 s^-1). In-vivo, upon laser-induced vascular injury, platelet recruitment and thrombus formation were markedly reduced in StfA1-overexpressing mice without affecting bleeding time. The presence of CA-074Me, a cathepsin B specific inhibitor significantly reduced thrombus formation in-vitro and in-vivo in human and mouse, respectively. Our study identifies StfA/CSTA as a key contributor of platelet-dependent thrombus formation in both rodents and humans.

The behavior of neutrophil extracellular traps and NADPH oxidative activity in pediatric systemic lupus erythematosus: relation to disease activity and lupus nephritis

OBJECTIVES

To evaluate the neutrophil extracellular traps (NETs) assay and NADPH oxidase (Nox2) activity in pediatric systemic lupus erythematosus (pSLE) in relation to each other and SLE characteristics.

METHODS

This cross-sectional study included 50 children and adolescents with pSLE who were clinically evaluated and underwent routine laboratory work up of SLE (CBC, ESR, 24 hrs urinary proteins, serum creatinine, complement-3 (C3), anti-dsDNA, and antiphospholipid antibodies). NETs assay and dihydrorhodamine (DHR) test were done for patient group and 50 age- and sex-matched control group.

RESULTS

The level of NETs was found significantly elevated among the patients (median 74.6 mU/ml) as compared to the controls (median 8.9 mU/ml) (p < 0.001), while values of DHR test were comparable between patients (median 95.5%) and controls (median 96.1%) (P = 0.55). There was a significant negative correlation between levels of NETs and DHR (p < 0.001). A significant positive correlation was noted between the 24 hrs urinary protein and NETs level (p < 0.001), but a significant negative correlation with DHR (p < 0.0001). Both NETs and DHR test values did not differ significantly between classes of lupus nephritis. NETs showed a significant positive correlation with anti-dsDNA titer (p = 0.004) and SLEDAI (p < 0.001), but a negative correlation with C3 (p < 0.001). DHR test was positively correlated with C3 levels (p = 0.003), but negatively correlated with anti-dsDNA titers (p = 0.008) and SLEDAI (p < 0.001).

CONCLUSION

NETs seem to have strong association with biomarkers of pSLE activity. On the other hand, Nox2 activity of the neutrophils was noted to be linked to quiescent state of SLE.

KEY POINTS

• Neutrophils have displayed different actions in pSLE through the NETs and Nox2 activity. • The inverse correlation between NETs and Nox2 activity makes the later a non-fundamental pathway for NETs formation. • NETs are associated with pSLE flare and LN activity, while neutrophil Nox2 activity is related to disease remission.

Autophagy preserves the osteogenic ability of periodontal ligament stem cells under high glucose conditions in rats

OBJECTIVE

To investigate how a high glucose environment influences the osteogenic ability of periodontal ligament stem cells (PDLSCs) and the function of autophagy in this process, we explored whether the osteogenic ability of PDLSCs could be protected by autophagy.

DESIGN

PDLSC proliferation and osteogenesis were evaluated by CCK-8 and western blotting under gradient glucose conditions. The Autophagy RT² Profiler PCR Array was used to screen autophagy-related mRNA expression during PDLSC osteoblastic differentiation on 5.5 mM + osteogenic induction (OI) medium or 25 mM + OI medium on day 3. Autophagy was regulated by an inducer (rapamycin) and inhibitor (bafilomycin) to investigate its protective effects on PDLSCs. A periodontal trauma model was established in diabetic rats to verify the effects of enhanced autophagy activity on PDLSCs.

RESULTS

A high glucose concentration (25 mM) impeded PDLSC proliferation on day 1, and compared with the control condition, high glucose also decreased the osteogenic ability of PDLSCs. The Autophagy RT² Profiler PCR Array showed obvious fluctuations in many autophagy-related genes, such as ULK1 (9.27), MTOR (3.15), MAP1LC3B (4.22), GABARAPL1 (7.09), ATG10 (6.5), AMPK14 (4.47), WIPI1 (3.29), and IGF1 (24.65). Compared with the control condition, an autophagy inducer or inhibitor markedly impaired or enhanced osteogenic differentiation in cells. The diabetic rat periodontal trauma model demonstrated that periodontium tissue partly recovered in the autophagy-enhanced cell injection diabetic rat group.

CONCLUSIONS

High glucose inhibited the activity of PDLSCs, and regulating autophagy protected cell function. Upregulating autophagy partially reversed the adverse effect of high glucose conditions on PDLSCs.

Enhancing autophagy protects platelets in immune thrombocytopenia patients

Background

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder and involves increased apoptosis of platelets. Autophagy is an essential process for platelets to maintain their life and physiological functions. However, the role of autophagy in ITP platelets was previously unclear.

Methods

In the present study, the expression of autophagy-related protein and autophagy flux were detected in platelets from ITP patients and healthy controls by immunofluorescence staining and immunoblotting, and the influence of autophagy on the viability and apoptosis of ITP platelets was further explored.

Results

We found that platelet autophagy was diminished in ITP patients. Platelet autophagy in ITP was regulated by the PI3K/AKT/mTOR pathway, with mTOR (mammalian target of rapamycin) as a negative regulator and class III PtdIns3K playing a crucial role in the process. Importantly, the small-molecule compound ABO (6-amino-2,3-dihydro-3-hydroxymethyl-1,4-benzoxazine) enhanced autophagy in ITP platelets. Enhancing platelet autophagy alleviated platelet destruction by inhibiting apoptosis and improving platelet viability.

Conclusions

These results suggest a role for autophagy regulation in the pathogenesis of ITP, and offer a novel treatment for these patients.

Megakaryocytic dysfunction in immune thrombocytopenia is linked to autophagy

Immune thrombocytopenic purpura (ITP) is a multifactorial autoimmune disease characterized by both increased platelet destruction and/or reduced platelet production. Even though they are detected in ≤ 50% of ITP patients, auto-antibodies play a pivotal role in the pathogenesis of ITP. Recent experimental and clinical observations have revealed abnormal autophagy in ITP patients. Autophagy is a catabolic process responsible for the elimination and recycling of cytoplasmic constituents, such as organelles and macromolecules, in eukaryotic cells. Additionally, it triggers cell death or promotes cell survival following various forms of stress, and maintains the microenvironment and stemness of haematopoietic stem cells. The role of autophagy in megakaryopoiesis, thrombopoiesis, and platelet function is slowly being uncovered. The abnormal autophagy in ITP patients may be caused by deletion of autophagy-related genes such as ATG7 and abnormal signalling due to overexpression of mTOR. These changes are thought to affect markers of haematopoietic stem cells, such as CD41 and CD61, and differentiation of megakaryocytes, ultimately decreasing the function and quantity of platelets and leading to the onset of ITP. This review highlights recent evidence on the essential role played by autophagy in megakaryopoiesis, megakaryocyte differentiation, thrombopoiesis, and platelet production. It also discusses the potential of targeting the autophagy pathway as a novel therapeutic approach against ITP.

Autophagy in Platelets

Anucleate platelets are produced by fragmentation of megakaryocytes. Platelets circulate in the bloodstream for a finite period: upon vessel injury, they are activated to participate in hemostasis; upon senescence, unused platelets are cleared. Platelet hypofunction leads to bleeding. Conversely, pathogenic platelet activation leads to occlusive events that precipitate strokes and heart attacks. Recently, we and others have shown that autophagy occurs in platelets and is important for platelet production and normal functions including hemostasis and thrombosis. Due to the unique properties of platelets, such as their lack of nuclei and their propensity for activation, methods for studying platelet autophagy must be specifically tailored. Here, we describe useful methods for examining autophagy in both human and mouse platelets.

Dissecting the biochemical architecture and morphological release pathways of the human platelet extracellular vesiculome

Platelet extracellular vesicles (PEVs) have emerged as potential mediators in intercellular communication. PEVs exhibit several activities with pathophysiological importance and may serve as diagnostic biomarkers. Here, imaging and analytical techniques were employed to unveil morphological pathways of the release, structure, composition, and surface properties of PEVs derived from human platelets (PLTs) activated with the thrombin receptor activating peptide (TRAP). Based on extensive electron microscopy analysis, we propose four morphological pathways for PEVs release from TRAP-activated PLTs: (1) plasma membrane budding, (2) extrusion of multivesicular α-granules and cytoplasmic vacuoles, (3) plasma membrane blistering and (4) "pearling" of PLT pseudopodia. The PLT extracellular vesiculome encompasses ectosomes, exosomes, free mitochondria, mitochondria-containing vesicles, "podiasomes" and PLT "ghosts". Interestingly, a flow cytometry showed a population of TOM20+LC3+ PEVs, likely products of platelet mitophagy. We found that lipidomic and proteomic profiles were different between the small PEV (S-PEVs; mean diameter 103 nm) and the large vesicle (L-PEVs; mean diameter 350 nm) fractions separated by differential centrifugation. In addition, the majority of PEVs released by activated PLTs was composed of S-PEVs which have markedly higher thrombin generation activity per unit of PEV surface area compared to L-PEVs, and contribute approximately 60% of the PLT vesiculome procoagulant potency.

Platelet Signaling and Disease: Targeted Therapy for Thrombosis and Other Related Diseases

Platelets are essential for clotting in the blood and maintenance of normal hemostasis. Under pathologic conditions such as atherosclerosis, vascular injury often results in hyperactive platelet activation, resulting in occlusive thrombus formation, myocardial infarction, and stroke. Recent work in the field has elucidated a number of platelet functions unique from that of maintaining hemostasis, including regulation of tumor growth and metastasis, inflammation, infection, and immune response. Traditional therapeutic targets for inhibiting platelet activation have primarily been limited to cyclooxygenase-1, integrin αIIbβ3, and the P2Y12 receptor. Recently identified signaling pathways regulating platelet function have made it possible to develop novel approaches for pharmacological intervention in the blood to limit platelet reactivity. In this review, we cover the newly discovered roles for platelets as well as their role in hemostasis and thrombosis. These new roles for platelets lend importance to the development of new therapies targeted to the platelet. Additionally, we highlight the promising receptor and enzymatic targets that may further decrease platelet activation and help to address the myriad of pathologic conditions now known to involve platelets without significant effects on hemostasis.

Aggregation is impaired in starved platelets due to enhanced autophagy and cellular energy depletion

Platelet hyperactivity is the hallmark of thrombosis and hemostasis disorders including atherosclerosis, diabetes, stroke, arthritis, and cancer causing significant mortality and morbidity. Therefore, regulating platelet hyperactivity is an ever growing interest. Very recently, basal autophagic process has been demonstrated to be essential for normal functioning of platelets. However, autophagy can be elevated above basal level under conditions like starvation, and how platelets respond in these settings remains to be elucidative. Therefore, in this study we demonstrate a substantial autophagy induction (above basal level) by starvation, which decreases platelet aggregation responses to various agonists. The decreased aggregation in starved platelets was restored in combination with autophagy inhibitors (3-methyladenine and NH₄Cl) and acetate supplementation. Starved platelets also showed decreased calcium mobilization, granule release, and adhesive properties. Furthermore, ex vivo platelets obtained from starved rats showed increased autophagy markers and decreased aggregation responses to various agonists. Our results distinctly explain that enhanced autophagy and cellular energy depletion are the cause for decreased platelet activation and aggregation. The study emphasizes the cardinal role of starvation and autophagy in the management of diseases and disorders associated with platelet hyperactivity.

Autophagy as a pharmacological target in hematopoiesis and hematological disorders

Autophagy is involved in many cellular processes, including cell homeostasis, cell death/survival balance and differentiation. Autophagy is essential for hematopoietic stem cell survival, quiescence, activation and differentiation. The deregulation of this process is associated with numerous hematological disorders and pathologies, including cancers. Thus, the use of autophagy modulators to induce or inhibit autophagy emerges as a potential therapeutic approach for treating these diseases and could be particularly interesting for differentiation therapy of leukemia cells. This review presents therapeutic strategies and pharmacological agents in the context of hematological disorders. The pros and cons of autophagy modulators in therapy will also be discussed.

Normal platelet function

Platelets play an important role in the vessel. Following their formation from megakaryocytes, platelets exist in circulation for 5-7 days and primarily function as regulators of hemostasis and thrombosis. Following vascular insult or injury, platelets become activated in the blood resulting in adhesion to the exposed extracellular matrix underlying the endothelium, formation of a platelet plug, and finally formation and consolidation of a thrombus consisting of both a core and shell. In pathological conditions, platelets are essential for formation of occlusive thrombus formation and as a result are the primary target for prevention of arterial thrombus formation. In addition to regulation of hemostasis in the vessel, platelets have also been shown to play an important role in innate immunity as well as regulation of tumor growth and extravasations in the vessel. These primary functions of the platelet represent its normal function and versatility in circulation.

Beyond COX-1: the effects of aspirin on platelet biology and potential mechanisms of chemoprevention

After more than a century, aspirin remains one of the most commonly used drugs in western medicine. Although mainly used for its anti-thrombotic, anti-pyretic, and analgesic properties, a multitude of clinical studies have provided convincing evidence that regular, low-dose aspirin use dramatically lowers the risk of cancer. These observations coincide with recent studies showing a functional relationship between platelets and tumors, suggesting that aspirin's chemopreventive properties may result, in part, from direct modulation of platelet biology and biochemistry. Here, we present a review of the biochemistry and pharmacology of aspirin with particular emphasis on its cyclooxygenase-dependent and cyclooxygenase-independent effects in platelets. We also correlate the results of proteomic-based studies of aspirin acetylation in eukaryotic cells with recent developments in platelet proteomics to identify non-cyclooxygenase targets of aspirin-mediated acetylation in platelets that may play a role in its chemopreventive mechanism.

Endogenous LINE-1 (Long Interspersed Nuclear Element-1) Reverse Transcriptase Activity in Platelets Controls Translational Events Through RNA-DNA Hybrids

OBJECTIVE

One source of endogenous reverse transcriptase (eRT) activity in nucleated cells is the LINE-1/L1 (long interspersed nuclear element-1), a non-LTR retrotransposon that is implicated in the regulation of gene expression. Nevertheless, the presence and function of eRT activity and LINE-1 in human platelets, an anucleate cell, has not previously been determined.

APPROACH AND RESULTS

We demonstrate that human and murine platelets possess robust eRT activity and identify the source as being LINE-1 ribonucleoprotein particles. Inhibition of eRT in vitro in isolated platelets from healthy individuals or in people with HIV treated with RT inhibitors enhanced global protein synthesis and platelet activation. If HIV patients were treated with reverse transcriptase inhibitor, we found that platelets from these patients had increased basal activation. We next discovered that eRT activity in platelets controlled the generation of RNA-DNA hybrids, which serve as translational repressors. Inhibition of platelet eRT lifted this RNA-DNA hybrid-induced translational block and was sufficient to increase protein expression of target RNAs identified by RNA-DNA hybrid immunoprecipitation.

CONCLUSIONS

Thus, we provide the first evidence that platelets possess L1-encoded eRT activity. We also demonstrate that platelet eRT activity regulates platelet hyperreactivity and thrombosis and controls RNA-DNA hybrid formation and identify that RNA-DNA hybrids function as a novel translational control mechanism in human platelets.

Mitophagy in Cardiomyocytes and in Platelets: A Major Mechanism of Cardioprotection Against Ischemia/Reperfusion Injury

Mitophagy, a process that selectively removes damaged organelles by autolysosomal degradation, is an early cellular response to ischemia. Mitophagy is activated in both cardiomyocytes and platelets during ischemia/reperfusion (I/R) and heart disease conditions. We focus on the molecular regulation of mitophagy and highlight the role of mitophagy in cardioprotection.

The ins and outs of endocytic trafficking in platelet functions

PURPOSE OF REVIEW

Although platelet endocytosis has been recognized in granule cargo loading and the trafficking of several platelet surface receptors, its acute physiological relevance is poorly understood as is its mechanism. The present review discusses the current understanding of platelet endocytosis and its implications for platelet function.

RECENT FINDINGS

Recent studies are beginning to identify and define the proteins that mediate platelet endocytosis. These studies have shown that platelets contain different endosomal compartments and may use multiple endocytic routes to take in circulating molecules and surface proteins. The studies have also shown that platelet endocytosis is involved in several aspects of platelet function such as signaling, spreading, and granule cargo loading.

SUMMARY

Mechanistic studies of platelet endocytosis have shown it to be not only involved in granule cargo loading but also in various other platelet functions important for hemostasis and beyond.

TRAF3 negatively regulates platelet activation and thrombosis

CD40 ligand (CD40L), a member of the tumor necrosis factor (TNF) superfamily, binds to CD40, leading to many effects depending on target cell type. Platelets express CD40L and are a major source of soluble CD40L. CD40L has been shown to potentiate platelet activation and thrombus formation, involving both CD40-dependent and -independent mechanisms. A family of proteins called TNF receptor associated factors (TRAFs) plays key roles in mediating CD40L-CD40 signaling. Platelets express several TRAFs. It has been shown that TRAF2 plays a role in CD40L-mediated platelet activation. Here we show that platelet also express TRAF3, which plays a negative role in regulating platelet activation. Thrombin- or collagen-induced platelet aggregation and secretion are increased in TRAF3 knockout mice. The expression levels of collagen receptor GPVI and integrin αIIbβ3 in platelets were not affected by deletion of TRAF3, suggesting that increased platelet activation in the TRAF3 knockout mice was not due to increased expression platelet receptors. Time to formation of thrombi in a FeCl₃-induced thrombosis model was significantly shortened in the TRAF3 knockout mice. However, mouse tail-bleeding times were not affected by deletion of TRAF3. Thus, TRAF3 plays a negative role in platelet activation and in thrombus formation in vivo.

A genome-wide association study links small-vessel ischemic stroke to autophagy

Genome-wide association studies (GWAS) can serve as strong evidence in correlating biological pathways with human diseases. Although ischemic stroke has been found to be associated with many biological pathways, the genetic mechanism of ischemic stroke is still unclear. Here, we performed GWAS for a major subtype of stroke-small-vessel occlusion (SVO)-to identify potential genetic factors contributing to ischemic stroke. GWAS were conducted on 342 individuals with SVO stroke and 1,731 controls from a Han Chinese population residing in Taiwan. The study was replicated in an independent Han Chinese population comprising an additional 188 SVO stroke cases and 1,265 controls. Three SNPs (rs2594966, rs2594973, rs4684776) clustered at 3p25.3 in ATG7 (encoding Autophagy Related 7), with P values between 2.52 × 10-6 and 3.59 × 10-6, were identified. Imputation analysis also supported the association between ATG7 and SVO stroke. To our knowledge, this is the first GWAS to link stroke and autophagy. ATG7, which has been implicated in autophagy, could provide novel insights into the genetic basis of ischemic stroke.

A dual role for the class III PI3K, Vps34, in platelet production and thrombus growth

To uncover the role of Vps34, the sole class III phosphoinositide 3-kinase (PI3K), in megakaryocytes (MKs) and platelets, we created a mouse model with Vps34 deletion in the MK/platelet lineage (Pf4-Cre/Vps34^lox/lox). Deletion of Vps34 in MKs led to the loss of its regulator protein, Vps15, and was associated with microthrombocytopenia and platelet granule abnormalities. Although Vps34 deficiency did not affect MK polyploidisation or proplatelet formation, it dampened MK granule biogenesis and directional migration toward an SDF1α gradient, leading to ectopic platelet release within the bone marrow. In MKs, the level of phosphatidylinositol 3-monophosphate (PI3P) was significantly reduced by Vps34 deletion, resulting in endocytic/trafficking defects. In platelets, the basal level of PI3P was only slightly affected by Vps34 loss, whereas the stimulation-dependent pool of PI3P was significantly decreased. Accordingly, a significant increase in the specific activity of Vps34 lipid kinase was observed after acute platelet stimulation. Similar to Vps34-deficient platelets, ex vivo treatment of wild-type mouse or human platelets with the Vps34-specific inhibitors, SAR405 and VPS34-IN1, induced abnormal secretion and affected thrombus growth at arterial shear rate, indicating a role for Vps34 kinase activity in platelet activation, independent from its role in MKs. In vivo, Vps34 deficiency had no impact on tail bleeding time, but significantly reduced platelet prothrombotic capacity after carotid injury. This study uncovers a dual role for Vps34 as a regulator of platelet production by MKs and as an unexpected regulator of platelet activation and arterial thrombus formation dynamics.

Class III PI3K Positively Regulates Platelet Activation and Thrombosis via PI(3)P-Directed Function of NADPH Oxidase

OBJECTIVE

Class III phosphoinositide 3-kinase, also known as VPS34 (vacuolar protein sorting 34), is a highly conserved enzyme regulating important cellular functions such as NADPH oxidase (NOX) assembly, membrane trafficking, and autophagy. Although VPS34 is expressed in platelets, its involvement in platelet activation remains unclear. Herein, we investigated the role of VPS34 in platelet activation and thrombus formation using VPS34 knockout mice.

APPROACH AND RESULTS

Platelet-specific VPS34-deficient mice were generated and characterized. VPS34 deficiency in platelets did not influence tail bleeding time. In a ferric chloride-induced mesenteric arteriolar thrombosis model, VPS34^-/- mice exhibited a prolonged vessel occlusion time compared with wild-type mice (42.05±4.09 versus 18.30±2.47 minutes). In an in vitro microfluidic whole-blood perfusion assay, thrombus formation on collagen under arterial shear was significantly reduced for VPS34^-/- platelets. VPS34^-/- platelets displayed an impaired aggregation and dense granule secretion in response to low doses of collagen or thrombin. VPS34 deficiency delayed clot retraction but did not influence platelet spreading on fibrinogen. We also demonstrated that VPS34 deficiency altered the basal level of autophagy in resting platelets and hampered NOX assembly and mTOR (mammalian target of rapamycin) signaling during platelet activation. Importantly, we identified the NOX-dependent reactive oxygen species generation as the major downstream effector of VPS34, which in turn can mediate platelet activation. In addition, by using a specific inhibitor 3-methyladenine, VPS34 was found to operate through a similar NOX-dependent mechanism to promote human platelet activation.

CONCLUSIONS

Platelet VPS34 is critical for thrombosis but dispensable for hemostasis. VPS34 regulates platelet activation by influencing NOX assembly.

Xanthohumol isolated from Humulus lupulus prevents thrombosis without increased bleeding risk by inhibiting platelet activation and mtDNA release

AIM

As the global population has reached 7 billion and the baby boom generation reaches old age, thrombosis has become the major contributor to the global disease burden. It has been reported that, in moderate doses, beer may protect against thrombosis. Xanthohumol (XN), an antioxidant, is found at high concentrations in hop cones (Humulus lupulus L.) and is a common ingredient of beer. Here, the aim of the present work was to investigate the effects of XN on antithrombotic and antiplatelet activities, and study its mechanism.

APPROACH AND RESULTS

Using ferric chloride-induced carotid artery injury, inferior vena cava ligation model, and platelet function tests, we demonstrated that XN uniquely prevents both venous and arterial thrombosis by inhibiting platelet activation. Interestingly, in tail bleeding time studies, XN did not increase bleeding risk, which is recognized as a major limitation of current antithrombotic therapies. We also demonstrated that XN induces Sirt1 expression and thereby decreases reactive oxygen species (ROS) overload, prevents mitochondrial dysfunction, and reduces activated platelet-induced mitochondrial hyperpolarization, respiratory disorders, and associated membrane damage at low concentrations. In mitochondrial function assays designed to detect amounts of extracellular mitochondrial DNA (mtDNA), we found that XN prevents mtDNA release, which induces platelet activation in a DC-SIGN-dependent manner.

CONCLUSIONS

XN exemplifies a promising new class of antiplatelet agents that are highly effective at inhibiting platelet activation by decreasing ROS accumulation and platelet mtDNA release without incurring a bleeding risk. This study has also provided novel insights into mechanisms of thrombotic diseases with possible therapeutic implications.

Standard Immunohistochemical Assays to Assess Autophagy in Mammalian Tissue

Autophagy is a highly conserved lysosomal degradation pathway with major impact on diverse human pathologies. Despite the development of different methodologies to detect autophagy both in vitro and in vivo, monitoring autophagy in tissue via immunohistochemical techniques is hampered due to the lack of biomarkers. Immunohistochemical detection of a punctate pattern of ATG8/MAP1LC3 proteins is currently the most frequently used approach to detect autophagy in situ, but it depends on a highly sensitive detection method and is prone to misinterpretation. Moreover, reliable MAP1LC3 immunohistochemical staining requires correct tissue processing and high-quality, isoform-specific antibodies. Immunohistochemical analysis of other autophagy-related protein targets such as SQSTM1, ubiquitin, ATG5 or lysosomal proteins is not recommended as marker for autophagic activity in tissue for multiple reasons including aspecific labeling of cellular structures and a lack of differential protein expression during autophagy initiation. To better understand the role of autophagy in human disease, novel biomarkers for visualization of the autophagic process with standard histology techniques are urgently needed.

Autophagy-related polymorphisms predict hypertension in patients with metastatic colorectal cancer treated with FOLFIRI and bevacizumab: Results from TRIBE and FIRE-3 trials

PURPOSE

The most frequent bevacizumab-related side-effects are hypertension, proteinuria, bleeding and thromboembolism. To date, there is no biomarker that predicts anti-VEGF-associated toxicity. As autophagy inhibits angiogenesis, we hypothesised that single-nucleotide polymorphisms (SNPs) within autophagy-related genes may predict bevacizumab-mediated toxicity in patients with metastatic colorectal cancer (mCRC).

PATIENTS AND METHODS

Patients with mCRC treated with first-line FOLFIRI and bevacizumab in two phase III randomised trials, namely the TRIBE trial (n = 219, discovery cohort) and the FIRE-3 trial (n = 234, validation cohort) were included in this study. Patients receiving treatment with FOLFIRI and cetuximab (FIRE-3, n = 204) served as a negative control. 12 SNPs in eight autophagy-related genes (ATG3/5/8/13, beclin 1, FIP200, unc-51-like kinase 1, UVRAG) were analysed by PCR-based direct sequencing.

RESULTS

The FIP200 rs1129660 variant showed significant associations with hypertension in the TRIBE cohort. Patients harbouring any G allele of the FIP200 rs1129660 SNP showed a significantly lower rate of grade 2-3 hypertension compared with the A/A genotype (3% versus 15%, odds ratio [OR] 0.17; 95% confidence interval [CI], 0.02-0.73; P = 0.009). Similarly, G allele carriers of the FIP200 rs1129660 SNP were less likely to develop grade 2-3 hypertension than patients with an A/A genotype in the FIRE-3 validation cohort (9% versus 20%, OR 0.43; 95% CI, 0.14-1.11; P = 0.077), whereas this association could not be observed in the control cohort (12% versus 9%, OR 1.40; 95% CI, 0.45-4.04; P = 0.60).

CONCLUSION

This is the first report demonstrating that polymorphisms in the autophagy-related FIP200 gene may predict hypertension in patients with mCRC treated with FOLFIRI and bevacizumab.

Endothelial-specific deletion of autophagy-related 7 (ATG7) attenuates arterial thrombosis in mice

BACKGROUND

Thrombosis persists as a leading cause of morbidity and mortality. Given that endothelial cells (ECs) play a central role in regulating thrombosis, understanding the molecular endothelial cues that regulate susceptibility or resistance to thrombosis have important translational implications. Accordingly, we evaluated the role of endothelial autophagy in the development of thrombosis.

METHODS

We generated mice in which the essential autophagy-related 7 (ATG7) gene was conditionally deleted from ECs (EC-ATG7^-/- mice). Three in vivo models of thrombosis were used, and mechanistic studies were conducted with cultured human umbilical vein endothelial cells (HUVECs).

RESULTS

We silenced ATG7 in HUVECs and observed >60% decreases in tumor necrosis factor (TNF)-α-induced tissue factor (TF) transcript levels, protein expression, and activity. TF mRNA levels in the carotid arteries of EC-ATG7^-/- mice subjected to the prothrombotic stimulus FeCl₃ were lower than those in the similarly treated wild-type (WT) littermate group. Compared with WT mice, EC-ATG7^-/- mice exhibited prolonged time to carotid (2-fold greater) and mesenteric (1.3-fold greater) artery occlusion following FeCl₃ injury. The thrombi generated in laser-injured cremasteric arterioles were smaller in EC-ATG7^-/- mice compared with WT mice, and took 2.3-fold longer to appear.

CONCLUSIONS

Taken together, these results provide definitive evidence that loss of endothelial ATG7 attenuates thrombosis and reduces the expression of TF. Our findings demonstrate that endothelial ATG7, and thus autophagy, is a critical and previously unrecognized target for modulating the susceptibility to thrombosis.

Neutrophil Extracellular Traps, Antiphospholipid Antibodies and Treatment

Neutrophil extracellular traps (NETs) are a network of extracellular fibers, compounds of chromatin, neutrophil DNA and histones, which are covered with antimicrobial enzymes with granular components. Autophagy and the production of reactive oxygen species (ROS) by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase are essential in the formation of NETs. There is increasing evidence that suggests that autoantibodies against beta-2-glycoprotein-1 (B2GP1) induce NETs and enhance thrombosis. Past research on new mechanisms of thrombosis formation in antiphospholipid syndrome (APS) has elucidated the pharmacokinetics of the most common medication in the treatment of the disease.

Hypoxic mitophagy regulates mitochondrial quality and platelet activation and determines severity of I/R heart injury

Mitochondrial dysfunction underlies many prevalent diseases including heart disease arising from acute ischemia/reperfusion (I/R) injury. Here, we demonstrate that mitophagy, which selectively removes damaged or unwanted mitochondria, regulated mitochondrial quality and quantity in vivo. Hypoxia induced extensive mitochondrial degradation in a FUNDC1-dependent manner in platelets, and this was blocked by in vivo administration of a cell-penetrating peptide encompassing the LIR motif of FUNDC1 only in wild-type mice. Genetic ablation of Fundc1 impaired mitochondrial quality and increased mitochondrial mass in platelets and rendered the platelets insensitive to hypoxia and the peptide. Moreover, hypoxic mitophagy in platelets protected the heart from worsening of I/R injury. This represents a new mechanism of the hypoxic preconditioning effect which reduces I/R injury. Our results demonstrate a critical role of mitophagy in mitochondrial quality control and platelet activation, and suggest that manipulation of mitophagy by hypoxia or pharmacological approaches may be a novel strategy for cardioprotection.

The nuts and bolts of the platelet release reaction

Secretion is essential to many of the roles that platelets play in the vasculature, e.g., thrombosis, angiogenesis, and inflammation, enabling platelets to modulate the microenvironment at sites of vascular lesions with a myriad of bioactive molecules stored in their granules. Past studies demonstrate that granule cargo release is mediated by Soluble NSF Attachment Protein Receptor (SNARE) proteins, which are required for granule-plasma membrane fusion. Several SNARE regulators, which control when, where, and how the SNAREs interact, have been identified in platelets. Additionally, platelet SNAREs are controlled by post-translational modifications, e.g., phosphorylation and acylation. Although there have been many recent insights into the mechanisms of platelet secretion, many questions remain: have we identified all the important regulators, does calcium directly control the process, and is platelet secretion polarized. In this review, we focus on the mechanics of platelet secretion and discuss how the secretory machinery functions in the pathway leading to membrane fusion and cargo release.

Human cells involved in atherosclerosis have a sex

The influence of sex has been largely described in cardiovascular diseases. Atherosclerosis is a complex process that involves many cell types such as vessel cells, immune cells and endothelial progenitor cells; however, many, if not all, studies do not report the sex of the cells. This review focuses on sex differences in human cells involved in the atherosclerotic process, emphasizing the role of sex hormones. Furthermore, we report sex differences and issues related to the processes that determine the fate of the cells such as apoptotic and autophagic mechanisms. The analysis of the data reveals that there are still many gaps in our knowledge regarding sex influences in atherosclerosis, largely for the cell types that have not been well studied, stressing the urgent need for a clear definition of experimental conditions and the inclusion of both sexes in preclinical studies.

Metformin Uniquely Prevents Thrombosis by Inhibiting Platelet Activation and mtDNA Release

Thrombosis and its complications are the leading cause of death in patients with diabetes. Metformin, a first-line therapy for type 2 diabetes, is the only drug demonstrated to reduce cardiovascular complications in diabetic patients. However, whether metformin can effectively prevent thrombosis and its potential mechanism of action is unknown. Here we show, metformin prevents both venous and arterial thrombosis with no significant prolonged bleeding time by inhibiting platelet activation and extracellular mitochondrial DNA (mtDNA) release. Specifically, metformin inhibits mitochondrial complex I and thereby protects mitochondrial function, reduces activated platelet-induced mitochondrial hyperpolarization, reactive oxygen species overload and associated membrane damage. In mitochondrial function assays designed to detect amounts of extracellular mtDNA, we found that metformin prevents mtDNA release. This study also demonstrated that mtDNA induces platelet activation through a DC-SIGN dependent pathway. Metformin exemplifies a promising new class of antiplatelet agents that are highly effective at inhibiting platelet activation by decreasing the release of free mtDNA, which induces platelet activation in a DC-SIGN-dependent manner. This study has established a novel therapeutic strategy and molecular target for thrombotic diseases, especially for thrombotic complications of diabetes mellitus.

Rapamycin and bafilomycin A1 alter autophagy and megakaryopoiesis

Autophagy is an effective strategy for cell development by recycling cytoplasmic constituents. Genetic deletion of autophagy mediator Atg7 in hematopoietic stem cells (HSCs) can lead to failure of megakaryopoiesis and enhanced autophagy has been implicated in various hematological disorders such as immune thrombocytopenia and myelodysplastic syndrome. Here, we examined the hypothesis that optimal autophagy is essential for megakaryopoiesis and thrombopoiesis by altering autophagy using pharmacological approaches. When autophagy was induced by rapamycin or inhibited by bafilomycin A1 in fetal liver cells, we observed a significant decrease in high ploidy megakaryocytes, a reduction of CD41 and CD61 co-expressing cells, and less proplatelet or platelet formation. Additionally, reduced cell size was shown in megakaryocytes derived from rapamycin, but not bafilomycin A1-treated mouse fetal liver cells. However, when autophagy was altered in mature megakaryocytes, we observed no significant change in proplatelet formation, which was consistent with normal platelet counts, megakaryocyte numbers, and ploidy in Atg7^flox/flox PF4-Cre mice with megakaryocyte- and platelet-specific deletion of autophagy-related gene Atg7. Therefore, our findings suggest that either induction or inhibition of autophagy in the early stage of megakaryopoiesis suppresses megakaryopoiesis and thrombopoiesis.

The prowess of platelets in immunity and inflammation

Platelets not only serve as essential haemostatic cells, they also have important roles in immune defence and inflammation. Despite not having a nucleus, platelets contain physiologically relevant amounts of RNA, which can be spliced and translated into functional proteins. In addition, platelets have the ability to bind to numerous other cells, such as leukocytes and vascular cells. During those interactions, platelets can modulate cellular responses, resulting in e. g. inflammatory activation or apoptosis. Recent studies have demonstrated that platelets can influence the outcomes of bacterial and viral infection, as well as the extent of tissue injury after ischaemia. Platelets also carry considerable amounts of cytokines and growth factors in their secretory granules, preformed for rapid secretion. Those properties in combination with the sheer amount of platelets circulating in the blood stream make them an important force in the immune response during health and disease. In this overview, recent findings concerning those interesting properties of platelets beyond haemostasis are discussed.

Inducing mitophagy in diabetic platelets protects against severe oxidative stress

Diabetes mellitus (DM) is a growing international concern. Considerable mortality and morbidity associated with diabetes mellitus arise predominantly from thrombotic cardiovascular events. Oxidative stress-mediated mitochondrial damage contributes significantly to enhanced thrombosis in DM A basal autophagy process has recently been described as playing an important role in normal platelet activation. We now report a substantial mitophagy induction (above basal autophagy levels) in diabetic platelets, suggesting alternative roles for autophagy in platelet pathology. Using a combination of molecular, biochemical, and imaging studies on human DM platelets, we report that platelet mitophagy induction serves as a platelet protective mechanism that responds to oxidative stress through JNK activation. By removing damaged mitochondria (mitophagy), phosphorylated p53 is reduced, preventing progression to apoptosis, and preserving platelet function. The absence of mitophagy in DM platelets results in failure to protect against oxidative stress, leading to increased thrombosis. Surprisingly, this removal of damaged mitochondria does not require contributions from transcription, as platelets lack a nucleus. The considerable energy and resources expended in "prepackaging" the complex mitophagy machinery in a short-lived normal platelet support a critical role, in anticipation of exposure to oxidative stress.

Role of autophagy in megakaryocyte differentiation and platelet formation

Autophagy is a conserved biological process for digestion and recycling of cytoplasmic constituents in eukaryotic cells. Autophagy may trigger cell death or promote cell survival following various forms of stress. The emerging roles of autophagy in megakaryopoiesis, thrombopoiesis, and platelet function have been uncovered using not only in vitro and in vivo genetic models, but also in clinical observations of autophagic structure in patients with thrombocytopenic disorders. Inhibition of autophagy in early stage of megakaryocyte differentiation appears to impede megakaryocyte maturation, reduce platelet formation, and affect platelet function, whereas autophagic deficiency in mature megakaryocytes gives rise to abnormal platelet activation and function without changing platelet size and number. On the other hand, induction of autophagy by rapamycin in megakaryocytes exhibited substantial therapeutic benefits in patients with immune thrombocytopenic purpura (ITP). This mini-review is to highlight recent progresses in understanding the regulation of autophagy in megakaryopoiesis, thrombopoiesis and platelet function to bridge the gap between autophagy and megakaryocyte/platelet pathophysiology.

Arf6 controls platelet spreading and clot retraction via integrin αIIbβ3 trafficking

Platelet and megakaryocyte endocytosis is important for loading certain granule cargo (ie, fibrinogen [Fg] and vascular endothelial growth factor); however, the mechanisms of platelet endocytosis and its functional acute effects are understudied. Adenosine 5'-diphosphate-ribosylation factor 6 (Arf6) is a small guanosine triphosphate-binding protein that regulates endocytic trafficking, especially of integrins. To study platelet endocytosis, we generated platelet-specific Arf6 knockout (KO) mice. Arf6 KO platelets had less associated Fg suggesting that Arf6 affects αIIbβ3-mediated Fg uptake and/or storage. Other cargo was unaffected. To measure Fg uptake, mice were injected with biotinylated- or fluorescein isothiocyanate (FITC)-labeled Fg. Platelets from the injected Arf6 KO mice showed lower accumulation of tagged Fg, suggesting an uptake defect. Ex vivo, Arf6 KO platelets were also defective in FITC-Fg uptake and storage. Immunofluorescence analysis showed initial trafficking of FITC-Fg to a Rab4-positive compartment followed by colocalization with Rab11-positive structures, suggesting that platelets contain and use both early and recycling endosomes. Resting and activated αIIbβ3 levels, as measured by flow cytometry, were unchanged; yet, Arf6 KO platelets exhibited enhanced spreading on Fg and faster clot retraction. This was not the result of alterations in αIIbβ3 signaling, because myosin light-chain phosphorylation and Rac1/RhoA activation were unaffected. Consistent with the enhanced clot retraction and spreading, Arf6 KO mice showed no deficits in tail bleeding or FeCl3-induced carotid injury assays. Our studies present the first mouse model for defining the functions of platelet endocytosis and suggest that altered integrin trafficking may affect the efficacy of platelet function.

The role of class I, II and III PI 3-kinases in platelet production and activation and their implication in thrombosis

Blood platelets play a pivotal role in haemostasis and are strongly involved in arterial thrombosis, a leading cause of death worldwide. Besides their critical role in pathophysiology, platelets represent a valuable model to investigate, both in vitro and in vivo, the biological roles of different branches of the phosphoinositide metabolism, which is highly active in platelets. While the phospholipase C (PLC) pathway has a crucial role in platelet activation, it is now well established that at least one class I phosphoinositide 3-kinase (PI3K) is also mandatory for proper platelet functions. Except class II PI3Kγ, all other isoforms of PI3Ks (class I α, β, γ, δ; class II α, β and class III) are expressed in platelets. Class I PI3Ks have been extensively studied in different models over the past few decades and several isoforms are promising drug targets to treat cancer and immune diseases. In platelet activation, it has been shown that while class I PI3Kδ plays a minor role, class I PI3Kβ has an important function particularly in thrombus growth and stability under high shear stress conditions found in stenotic arteries. This class I PI3K is a potentially interesting target for antithrombotic strategies. The role of class I PI3Kα remains ill defined in platelets. Herein, we will discuss our recent data showing the potential impact of inhibitors of this kinase on thrombus formation. The role of class II PI3Kα and β as well as class III PI3K (Vps34) in platelet production and function is just emerging. Based on our data and those very recently published in the literature, we will discuss the impact of these three PI3K isoforms in platelet production and functions and in thrombosis.