Endobrevin/VAMP-8-dependent dense granule release mediates thrombus formation in vivo

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

Tyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4-mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5'-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4-mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

Tyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4-mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5'-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4-mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

Release of α-granule contents during platelet activation

Upon activation, platelets release a plethora of factors which help to mediate their dynamic functions in hemostasis, inflammation, wound healing, tumor metastasis and angiogenesis. The majority of these bioactive molecules are released from α-granules, which are unique to platelets, and contain an incredibly diverse repertoire of cargo including; integral membrane proteins, pro-coagulant molecules, chemokines, mitogenic, growth and angiogenic factors, adhesion proteins, and microbicidal proteins. Clinically, activation of circulating platelets has increasingly been associated with various disease states. Biomarkers indicating the level of platelet activation in patients can therefore be useful tools to evaluate risk factors to predict future complications and determine treatment strategies or evaluate antiplatelet therapy. The irreversible nature of α-granule secretion makes it ideally suited as a marker of platelet activation. This review outlines the release and contents of platelet α-granules, as well as the membrane bound, and soluble α-granule cargo proteins that can be used as biomarkers of platelet activation.

The Provocative Roles of Platelets in Liver Disease and Cancer

Both platelets and the liver play important roles in the processes of coagulation and innate immunity. Platelet responses at the site of an injury are rapid; their immediate activation and structural changes minimize the loss of blood. The majority of coagulation proteins are produced by the liver—a multifunctional organ that also plays a critical role in many processes: removal of toxins and metabolism of fats, proteins, carbohydrates, and drugs. Chronic inflammation, trauma, or other causes of irreversible damage to the liver can dysregulate these pathways leading to organ and systemic abnormalities. In some cases, platelet-to-lymphocyte ratios can also be a predictor of disease outcome. An example is cirrhosis, which increases the risk of bleeding and prothrombotic events followed by activation of platelets. Along with a triggered coagulation cascade, the platelets increase the risk of pro-thrombotic events and contribute to cancer progression and metastasis. This progression and the resulting tissue destruction is physiologically comparable to a persistent, chronic wound. Various cancers, including colorectal cancer, have been associated with increased thrombocytosis, platelet activation, platelet-storage granule release, and thrombosis; anti-platelet agents can reduce cancer risk and progression. However, in cancer patients with pre-existing liver disease who are undergoing chemotherapy, the risk of thrombotic events becomes challenging to manage due to their inherent risk for bleeding. Chemotherapy, also known to induce damage to the liver, further increases the frequency of thrombotic events. Depending on individual patient risks, these factors acting together can disrupt the fragile balance between pro- and anti-coagulant processes, heightening liver thrombogenesis, and possibly providing a niche for circulating tumor cells to adhere to—thus promoting both liver metastasis and cancer-cell survival following treatment (that is, with minimal residual disease in the liver).

Mechanism of platelet α-granule biogenesis: study of cargo transport and the VPS33B-VPS16B complex in a model system

Platelet α-granules play important roles in platelet function. They contain hundreds of proteins that are synthesized by the megakaryocyte or taken up by endocytosis. The trafficking pathways that mediate platelet α-granule biogenesis are incompletely understood, especially with regard to cargo synthesized by the megakaryocyte. Vacuolar-protein sorting 33B (VPS33B) and VPS16B are essential proteins for α-granule biogenesis, but they are largely uncharacterized. Here, we adapted a powerful method to directly map the pathway followed by newly synthesized cargo proteins to reach α-granules. Using this method, we revealed the recycling endosome as a key intermediate compartment in α-granule biogenesis. We then used CRISPR/Cas9 gene editing to knock out VPS33B in pluripotent stem cell-derived immortalized megakaryocyte cells (imMKCLs). Consistent with the observations in platelets from patients with VPS33B mutation, VPS33B-knockout (KO) imMKCLs have drastically reduced levels of α-granule proteins platelet factor 4, von Willebrand factor, and P-selectin. VPS33B and VPS16B form a distinct and small complex in imMKCLs with the same hydrodynamic radius as the recombinant VPS33B-VPS16B heterodimer purified from bacteria. Mechanistically, the VPS33B-VPS16B complex ensures the correct trafficking of α-granule proteins. VPS33B deficiency results in α-granule cargo degradation in lysosomes. VPS16B steady-state levels are significantly lower in VPS33B-KO imMKCLs, suggesting that VPS16B is destabilized in the absence of its partner. Exogenous expression of green fluorescent protein-VPS33B in VPS33B-KO imMKCLs reconstitutes the complex, which localizes to the recycling endosome, further defining this compartment as a key intermediate in α-granule biogenesis. These results advance our understanding of platelet α-granule biogenesis and open new avenues for the study of these organelles.

Platelet α-granules are required for occlusive high-shear-rate thrombosis

von Willebrand factor (VWF) is essential for the induction of arterial thrombosis. In this study, we investigated the critical role of platelet VWF in occlusive thrombosis formation at high shear in mice that do not express platelet VWF (Nbeal2-/-). Using in silico modeling, in vitro high-shear microfluidics, and an in vivo Folts model of arterial thrombosis we reproduced the platelet dynamics that occur under pathological flow in a stenosed vessel. Computational fluid dynamics (CFDs) simulated local hemodynamics in a stenosis based on arterial geometries. The model predicted shear rates, time course of platelet adhesion, and time to occlusion. These predictions were validated in vitro and in vivo. Occlusive thrombosis developed in wild-type control mice that had normal levels of plasma VWF and platelet VWF in vitro and in vivo. Occlusive thrombosis did not form in the Nbeal2-/- mice that had normal plasma VWF and an absence of platelet VWF. Occlusive thrombosis was corrected in Nbeal2-/- microfluidic assays by the addition of exogenous normal platelets with VWF. Combining model and experimental data, we demonstrated the necessary requirement of platelet VWF in α-granules in forming an occlusive thrombus under high shear. These results could inspire new pharmacological targets specific to pathological conditions and prevent arterial thrombosis.

Mouse laser injury models: variations on a theme

A confluence of technological advances in genetic manipulation and molecular-based fluorescence imaging has led to the widespread adoption of laser injury models to study hemostasis and thrombosis in mice. In all animal models of hemostasis and thrombosis, detailing the nature of experimentally induced vascular injury is paramount in enabling appropriate interpretation of experimental results. A careful appraisal of the literature shows that direct laser-induced injury can result in variable degrees of vascular damage. This review will compare and contrast models of laser injury utilized in the field, with an emphasis on the mechanism and extent of injury, the use of laser injury in different vascular beds and the molecular mechanisms regulating the response to injury. All of these topics will be discussed in the context of how distinct applications of laser injury models may be viewed as representing thrombosis and/or hemostasis.

Our First Experience with Magnetic Separation of Platelets for Analyses of Platelet MicroRNA in Patients with Sticky Platelet Syndrome

Introduction: Sticky platelet syndrome (SPS) is referred to as a platelet hyperaggregability triggered by low concentrations of platelet agonists adenosine diphosphate (ADP) and/or epinephrine (EPI). Platelet aggregation with other inducers (collagen, arachidonic acid, ristocetin, and thrombin) remains within a normal range. MicroRNAs (miRNAs) are small, non-coding RNA molecules that play an important role in post-transcriptional regulation of protein expression. More recently, several studies show that the platelets are an abundant source of miRNAs and that the miRNA expression profiles within platelets correlate with the platelet reactivity.

Aim: The principle objective of this article is to describe the method which we developed for the preparation of the pure platelet samples and report the results of this method. These final pure platelet samples are intended to be the first step for the platelet miRNA testing.

Methods: The blood samples from 50 subjects were examined in the study. Then, the platelet rich plasma (PRP) samples obtained by centrifugation of the patient blood samples were used for our experiments. Subsequently, the erythrocytes and leucocytes remaining in PRP sample were magnetically labelled by CD45 Microbeads and CD235a Microbeads. After incubation the PRP sample passed through the magnetic separation system and the magnetically labelled cells (erythrocytes and leucocytes) were retained within the column of separator. The number of cells in the final PRP samples was measured by the blood cell analyser.

Results and conclusion: We successfully developed and optimized the effective and reproducible method for magnetic separation of platelets, resulting in the leukocyte-depleted and erythrocyte-depleted platelet samples, which can be used for further genetic analyses.

Defective AP-3-dependent VAMP8 trafficking impairs Weibel-Palade body exocytosis in Hermansky-Pudlak Syndrome type 2 blood outgrowth endothelial cells

Weibel-Palade bodies are endothelial secretory organelles that contain von Willebrand factor, P-selectin and CD63. Release of von Willebrand factor from Weibel-Palade bodies is crucial for platelet adhesion during primary hemostasis. Endosomal trafficking of proteins like CD63 to Weibel-Palade bodies during maturation is dependent on the adaptor protein complex 3 complex. Mutations in the AP3B1 gene, which encodes the adaptor protein complex 3 β1 subunit, result in Hermansky-Pudlak syndrome 2, a rare genetic disorder that leads to neutropenia and a mild bleeding diathesis. This is caused by abnormal granule formation in neutrophils and platelets due to defects in trafficking of cargo to secretory organelles. The impact of these defects on the secretory pathway of the endothelium is largely unknown. In this study, we investigated the role of adaptor protein complex 3-dependent mechanisms in trafficking of proteins during Weibel-Palade body maturation in endothelial cells. An ex vivo patient-derived endothelial model of Hermansky-Pudlak syndrome type 2 was established using blood outgrowth endothelial cells that were isolated from a patient with compound heterozygous mutations in AP3B1 Hermansky-Pudlak syndrome type 2 endothelial cells and CRISPR-Cas9-engineered AP3B1-/- endothelial cells contain Weibel-Palade bodies that are entirely devoid of CD63, indicative of disrupted endosomal trafficking. Hermansky-Pudlak syndrome type 2 endothelial cells have impaired Ca2+-mediated and cAMP-mediated exocytosis. Whole proteome analysis revealed that, apart from adaptor protein complex 3 β1, also the μ1 subunit and the v-SNARE VAMP8 were depleted. Stimulus-induced von Willebrand factor secretion was impaired in CRISPR-Cas9-engineered VAMP8-/-endothelial cells. Our data show that defects in adaptor protein complex 3-dependent maturation of Weibel-Palade bodies impairs exocytosis by affecting the recruitment of VAMP8.

Alterations in platelet secretion differentially affect thrombosis and hemostasis

We genetically manipulated the major platelet vesicle-associated membrane proteins (VAMP2, VAMP3, and VAMP8) to create mice with varying degrees of disrupted platelet secretion. As previously shown, loss of VAMP8 reduced granule secretion, and this defect was exacerbated by further deletion of VAMP2 and VAMP3. VAMP2^Δ3^Δ8^-/- platelets also had reduced VAMP7. Loss of VAMP2 and VAMP3 (VAMP2^Δ3^Δ) had a minimal impact on secretion when VAMP7 and VAMP8 were present. Integrin αIIbβ3 activation and aggregation were not affected, although spreading was reduced in VAMP2^Δ3^Δ8^-/- platelets. Using these mice as tools, we asked how much secretion is needed for proper thrombosis and hemostasis in vivo. VAMP2^Δ3^Δ mice showed no deficiency, whereas VAMP8^-/- mice had attenuated formation of occlusive thrombi upon FeCl₃-induced arterial injury but no excessive bleeding upon tail transection. VAMP2^Δ3^Δ8^-/- mice bled profusely and failed to form occlusive thrombi. Plasma-coagulation factors were normal in all of the strains, but phosphatidylserine exposure was reduced in VAMP2^Δ3^Δ and VAMP2^Δ3^Δ8^-/- platelets. From our data, an ∼40% to 50% reduction in platelet secretion in vitro (dense and α granule) correlated with reduced occlusive thrombosis but no compromise in hemostasis. At a >50% reduction, thrombosis and hemostasis were defective in vivo. Our studies are the first systematic manipulation of platelet exocytic machinery to demonstrate a quantitative linkage between in vitro platelet secretion and hemostasis and thrombosis in vivo. The animals described will be invaluable tools for future investigations into how platelet secretion affects other vascular processes.

Impact of Tissue Factor Localization on Blood Clot Structure and Resistance under Venous Shear

The structure and growth of a blood clot depend on the localization of tissue factor (TF), which can trigger clotting during the hemostatic process or promote thrombosis when exposed to blood under pathological conditions. We sought to understand how the growth, structure, and mechanical properties of clots under flow are shaped by the simultaneously varying TF surface density and its exposure area. We used an eight-channel microfluidic device equipped with a 20- or 100-μm-long collagen surface patterned with lipidated TF of surface densities ∼0.1 and ∼2 molecules/μm². Human whole blood was perfused at venous shear, and clot growth was continually measured. Using our recently developed computational model of clot formation, we performed simulations to gain insights into the clot’s structure and its resistance to blood flow. An increase in TF exposure area resulted not only in accelerated bulk platelet, thrombin, and fibrin accumulation, but also in increased height of the platelet mass and increased clot resistance to flow. Moreover, increasing the TF surface density or exposure area enhanced platelet deposition by approximately twofold, and thrombin and fibrin generation by greater than threefold, thereby increasing both clot size and its viscous resistance. Finally, TF effects on blood flow occlusion were more pronounced for the longer thrombogenic surface than for the shorter one. Our results suggest that TF surface density and its exposure area can independently enhance both the clot’s occlusivity and its resistance to blood flow. These findings provide, to our knowledge, new insights into how TF affects thrombus growth in time and space under flow.

Munc18-2, but not Munc18-1 or Munc18-3, regulates platelet exocytosis, hemostasis, and thrombosis

Platelet degranulation, a form of regulated exocytosis, is crucial for hemostasis and thrombosis. Exocytosis in platelets is mediated by SNARE proteins, and in most mammalian cells this process is controlled by Munc18 (mammalian homolog of Caenorhabditis elegans uncoordinated gene 18) proteins. Platelets express all Munc18 paralogs (Munc18-1, -2, and -3), but their roles in platelet secretion and function have not been fully characterized. Using Munc18-1, -2, and -3 conditional knockout mice, here we deleted expression of these proteins in platelets and assessed granule exocytosis. We measured products secreted by each type of platelet granule and analyzed EM platelet profiles by design-based stereology. We observed that the removal of Munc18-2 ablates the release of alpha, dense, and lysosomal granules from platelets, but we found no exocytic role for Munc18-1 or -3 in platelets. In vitro, Munc18-2-deficient platelets exhibited defective aggregation at low doses of collagen and impaired thrombus formation under shear stress. In vivo, megakaryocyte-specific Munc18-2 conditional knockout mice had a severe hemostatic defect and prolonged arterial and venous bleeding times. They were also protected against arterial thrombosis in a chemically induced model of arterial injury. Taken together, our results indicate that Munc18-2, but not Munc18-1 or Munc18-3, is essential for regulated exocytosis in platelets and platelet participation in thrombosis and hemostasis.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Platelet-Enriched MicroRNAs and Cardiovascular Homeostasis

SIGNIFICANCE

Platelets are anucleate blood cells that are involved in hemostasis and thrombosis. Although no longer able to generate ribonucleic acid (RNA) de novo, platelets contain messenger RNA (mRNA), YRNA fragments, and premature microRNAs (miRNAs) that they inherit from megakaryocytes. Recent Advances: Novel sequencing techniques have helped identify the unexpectedly large number of RNA species present in platelets. Throughout their life time, platelets can process the pre-existing pool of premature miRNA to give the fully functional miRNA that can regulate platelet protein expression and function.

CRITICAL ISSUES

Platelets make a major contribution to the circulating miRNA pool but platelet activation can have major consequences on Dicer levels and thus miRNA maturation, which has implications for studies that are focused on screening-stored platelets.

FUTURE DIRECTIONS

It will be important to determine the importance of platelets as donors for miRNA-containing microvesicles that can be taken up and processed by other (particularly vascular) cells, thus contributing to homeostasis as well as disease progression. Antioxid. Redox Signal. 29, 902-921.

AMPK-ACC signaling modulates platelet phospholipids and potentiates thrombus formation

AMP-activated protein kinase (AMPK) α1 is activated in platelets on thrombin or collagen stimulation, and as a consequence, phosphorylates and inhibits acetyl-CoA carboxylase (ACC). Because ACC is crucial for the synthesis of fatty acids, which are essential for platelet activation, we hypothesized that this enzyme plays a central regulatory role in platelet function. To investigate this, we used a double knock-in (DKI) mouse model in which the AMPK phosphorylation sites Ser79 on ACC1 and Ser212 on ACC2 were mutated to prevent AMPK signaling to ACC. Suppression of ACC phosphorylation promoted injury-induced arterial thrombosis in vivo and enhanced thrombus growth ex vivo on collagen-coated surfaces under flow. After collagen stimulation, loss of AMPK-ACC signaling was associated with amplified thromboxane generation and dense granule secretion. ACC DKI platelets had increased arachidonic acid-containing phosphatidylethanolamine plasmalogen lipids. In conclusion, AMPK-ACC signaling is coupled to the control of thrombosis by specifically modulating thromboxane and granule release in response to collagen. It appears to achieve this by increasing platelet phospholipid content required for the generation of arachidonic acid, a key mediator of platelet activation.

microRNAs in the haemostatic system: More than witnesses of thromboembolic diseases?

MicroRNAs (miRNAs) are small endogenous RNAs that post-transcriptionally regulate gene expression. In the last few years, these molecules have been implicated in the regulation of haemostasis, and an increasing number of studies have investigated their relationship with the development of thrombosis. In this review, we discuss the latest developments regarding the role of miRNAs in the regulation of platelet function and secondary haemostasis. We also discuss the genetic and environmental factors that regulate miRNAs. Finally, we address the potential use of miRNAs as prognostic and diagnostic tools in thrombosis.

Dynamic cycling of t-SNARE acylation regulates platelet exocytosis

Platelets regulate vascular integrity by secreting a host of molecules that promote hemostasis and its sequelae. Given the importance of platelet exocytosis, it is critical to understand how it is controlled. The t-SNAREs, SNAP-23 and syntaxin-11, lack classical transmembrane domains (TMDs), yet both are associated with platelet membranes and redistributed into cholesterol-dependent lipid rafts when platelets are activated. Using metabolic labeling and hydroxylamine (HA)/HCl treatment, we showed that both contain thioester-linked acyl groups. Mass spectrometry mapping further showed that syntaxin-11 was modified on cysteine 275, 279, 280, 282, 283, and 285, and SNAP-23 was modified on cysteine 79, 80, 83, 85, and 87. Interestingly, metabolic labeling studies showed incorporation of [3H]palmitate into the t-SNAREs increased although the protein levels were unchanged, suggesting that acylation turns over on the two t-SNAREs in resting platelets. Exogenously added fatty acids did compete with [3H]palmitate for t-SNARE labeling. To determine the effects of acylation, we measured aggregation, ADP/ATP release, as well as P-selectin exposure in platelets treated with the acyltransferase inhibitor cerulenin or the thioesterase inhibitor palmostatin B. We found that cerulenin pretreatment inhibited t-SNARE acylation and platelet function in a dose- and time-dependent manner whereas palmostatin B had no detectable effect. Interestingly, pretreatment with palmostatin B blocked the inhibitory effects of cerulenin, suggesting that maintaining the acylation state is important for platelet function. Thus, our work shows that t-SNARE acylation is actively cycling in platelets and suggests that the enzymes regulating protein acylation could be potential targets to control platelet exocytosis in vivo.

Tie2 protects the vasculature against thrombus formation in systemic inflammation

Disordered coagulation contributes to death in sepsis and lacks effective treatments. Existing markers of disseminated intravascular coagulation (DIC) reflect its sequelae rather than its causes, delaying diagnosis and treatment. Here we show that disruption of the endothelial Tie2 axis is a sentinel event in septic DIC. Proteomics in septic DIC patients revealed a network involving inflammation and coagulation with the Tie2 antagonist, angiopoietin-2 (Angpt-2), occupying a central node. Angpt-2 was strongly associated with traditional DIC markers including platelet counts, yet more accurately predicted mortality in 2 large independent cohorts (combined N = 1,077). In endotoxemic mice, reduced Tie2 signaling preceded signs of overt DIC. During this early phase, intravital imaging of microvascular injury revealed excessive fibrin accumulation, a pattern remarkably mimicked by Tie2 deficiency even without inflammation. Conversely, Tie2 activation normalized prothrombotic responses by inhibiting endothelial tissue factor and phosphatidylserine exposure. Critically, Tie2 activation had no adverse effects on bleeding. These results mechanistically implicate Tie2 signaling as a central regulator of microvascular thrombus formation in septic DIC and indicate that circulating markers of the Tie2 axis could facilitate earlier diagnosis. Finally, interventions targeting Tie2 may normalize coagulation in inflammatory states while averting the bleeding risks of current DIC therapies.

The life cycle of platelet granules

Platelet granules are unique among secretory vesicles in both their content and their life cycle. Platelets contain three major granule types—dense granules, α-granules, and lysosomes—although other granule types have been reported. Dense granules and α-granules are the most well-studied and the most physiologically important. Platelet granules are formed in large, multilobulated cells, termed megakaryocytes, prior to transport into platelets. The biogenesis of dense granules and α-granules involves common but also distinct pathways. Both are formed from the trans-Golgi network and early endosomes and mature in multivesicular bodies, but the formation of dense granules requires trafficking machinery different from that of α-granules. Following formation in the megakaryocyte body, both granule types are transported through and mature in long proplatelet extensions prior to the release of nascent platelets into the bloodstream. Granules remain stored in circulating platelets until platelet activation triggers the exocytosis of their contents. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, located on both the granules and target membranes, provide the mechanical energy that enables membrane fusion during both granulogenesis and exocytosis. The function of these core fusion engines is controlled by SNARE regulators, which direct the site, timing, and extent to which these SNAREs interact and consequently the resulting membrane fusion. In this review, we assess new developments in the study of platelet granules, from their generation to their exocytosis.

Complement Activation in Trauma Patients Alters Platelet Function

Trauma remains the main cause of death for both civilians and those in uniform. Trauma-associated coagulopathy is a complex process involving inflammation, coagulation, and platelet dysfunction. It is unknown whether activation of complement, which occurs invariably in trauma patients, is involved in the expression of trauma-associated coagulopathy. We designed a prospective study in which we enrolled 40 trauma patients and 30 healthy donors upon arrival to the emergency department of BIDMC. Platelets from healthy individuals were incubated with sera from trauma patients and their responsiveness to a thrombin receptor-activating peptide was measured using aggregometry. Complement deposition on platelets from trauma patients was measured by flow cytometry. Normal platelets displayed hypoactivity after incubation with trauma sera even though exposure to trauma sera resulted in increased agonist-induced calcium flux. Depletion of complement from sera further blocked activation of hypoactive platelets. Conversely, complement activation increased aggregation of platelets. Platelets from trauma patients were found to have significantly higher amounts of C3a and C4d on their surface compared with platelets from controls. Depletion of complement (C4d, C3a) reversed the ability of trauma sera to augment agonist-induced calcium flux in donor platelets. Our data indicate that complement enhances platelet aggregation. Despite its complement content, trauma sera render platelets hypoactive and complement depletion further blocks activation of hypoactive platelets. The defect in platelet activation induced by trauma sera is distal to receptor activation since agonist-induced Ca2+ flux is elevated in the presence of trauma sera owing to complement deposition.

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.

Platelet dense granules begin to selectively accumulate mepacrine during proplatelet formation

Platelet dense granules (DGs) are storage organelles for calcium ions, small organic molecules such as ADP and serotonin, and larger polyphosphates that are secreted upon platelet stimulation to enhance platelet activation, adhesion, and stabilization at sites of vascular damage. DGs are thought to fully mature within megakaryocytes (MKs) prior to platelet formation. Here we challenge this notion by exploiting vital fluorescent dyes to distinguish mildly acidic DGs from highly acidic compartments by microscopy in platelets and MKs. In isolated primary mouse platelets, compartments labeled by mepacrine - a fluorescent weak base that accumulates in DGs - are readily distinguishable from highly acidic compartments, likely lysosomes, that are labeled by the acidic pH indicator, LysoTracker, and from endolysosomes and alpha granules labeled by internalized and partially digested DQ™ BSA. By contrast, in murine fetal liver- and human CD34⁺ cell-derived MKs and the megakaryocytoid cell lines, MEG-01 and differentiated G1ME2, labeling by mepacrine overlapped nearly completely with labeling by LysoTracker and partially with labeling by DQ™ BSA. Mepacrine labeling in G1ME2-derived MKs was fully sensitive to proton ATPase inhibitors, but was only partially sensitive in platelets. These data indicate that mepacrine in MKs accumulates as a weak base in endolysosomes but is likely pumped into or retained in separate DGs in platelets. Fluorescent puncta that labeled uniquely for mepacrine were first evident in G1ME2-derived proplatelets, suggesting that DGs undergo a maturation step that initiates in the final stages of MK differentiation.

Platelet secretion in inflammatory and infectious diseases

Platelets are small, anucleate circulating cells that possess a dynamic repertoire of functions spanning the hemostatic, inflammatory, and immune continuum. Once thought to be merely cell fragments with responses limited primarily to acute hemostasis and vascular wall repair, platelets are now increasingly recognized as key sentinels and effector cells regulating host responses to many inflammatory and infectious cues. Platelet granules, including α-granules and dense-granules, store hundreds of factors and secrete these mediators in response to activating signals. The cargo packaged and stored within platelet granules orchestrates communication between platelets and other circulating cells, augments host defense mechanisms to invading pathogens and tumor cells, and - in some settings - drives dysregulated and injurious responses. This focused review will highlight several of the established and emerging mechanisms and roles of platelet secretion in inflammatory and infectious diseases.

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.

Storage pool diseases illuminate platelet dense granule biogenesis

Platelet dense granules (DGs) are membrane bound compartments that store polyphosphate and small molecules such as ADP, ATP, Ca²⁺, and serotonin. The release of DG contents plays a central role in platelet aggregation to form a hemostatic plug. Accordingly, congenital deficiencies in the biogenesis of platelet DGs underlie human genetic disorders that cause storage pool disease and manifest with prolonged bleeding. DGs belong to a family of lysosome-related organelles, which also includes melanosomes, the compartments where the melanin pigments are synthesized. These organelles share several characteristics including an acidic lumen and, at least in part, the molecular machinery involved in their biogenesis. As a result, many genes affect both DG and melanosome biogenesis and the corresponding patients present not only with bleeding but also with oculocutaneous albinism. The identification and characterization of such genes has been instrumental in dissecting the pathways responsible for organelle biogenesis. Because the study of melanosome biogenesis has advanced more rapidly, this knowledge has been extrapolated to explain how DGs are produced. However, some progress has recently been made in studying platelet DG biogenesis directly in megakaryocytes and megakaryocytoid cells. DGs originate from an endosomal intermediate compartment, the multivesicular body. Maturation and differentiation into a DG begins when newly synthesized DG-specific proteins are delivered from early/recycling endosomal compartments. The machinery that orchestrates this vesicular trafficking is composed of a combination of both ubiquitous and cell type-specific proteins. Here, we review the current knowledge on DG biogenesis. In particular, we focus on the individual human and murine genes encoding the molecular machinery involved in this process and how their deficiencies result in disease.

VAMP2 is implicated in the secretion of antibodies by human plasma cells and can be replaced by other synaptobrevins

The production and secretion of antibodies by human plasma cells (PCs) are two essential processes of humoral immunity. The secretion process relies on a group of proteins known as soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are located in the plasma membrane (t-SNAREs) and in the antibody-carrying vesicle membrane (v-SNARE), and mediate the fusion of both membranes. We have previously shown that SNAP23 and STX4 are the t-SNAREs responsible for antibody secretion. Here, using human PCs and antibody-secreting cell lines, we studied and characterized the expression and subcellular distribution of vesicle associated membrane protein (VAMP) isoforms, demonstrating that all isoforms (with the exception of VAMP1) are expressed by the referenced cells. Furthermore, the functional role in antibody secretion of each expressed VAMP isoform was tested using siRNA. Our results show that VAMP2 may be the v-SNARE involved in vesicular antibody release. To further support this conclusion, we used tetanus toxin light chain to cleave VAMP2, conducted experiments to verify co-localization of VAMP2 in antibody-carrying vesicles, and demonstrated the coimmunoprecipitation of VAMP2 with STX4 and SNAP23 and the in situ interaction of VAMP2 with STX4. Taken together, these findings implicate VAMP2 as the main VAMP isoform functionally involved in antibody secretion.

Integrin-αIIbβ3-mediated outside-in signalling activates a negative feedback pathway to suppress platelet activation

Integrin-αIIbβ3-mediated outside-in signalling is widely accepted as an amplifier of platelet activation; accumulating evidence suggests that outside-in signalling can, under certain conditions, also function as an inhibitor of platelet activation. The role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation is disputable. We employed flow cytometry, aggregometry, immunoprecipitation, and immunoblotting to investigate the role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation. Integrin αIIbβ3 inhibition enhances agonist-induced platelet ATP secretion. Human platelets lacking expression of αIIbβ3 exhibited more platelet ATP secretion than their wild-type counterparts. Moreover, integrin-αIIbβ3-mediated outside-in signals activate SHIP-1, which in turn mediates p-Akt dephosphorylation, leading to inactivation of PI3K/Akt signalling. Furthermore, 3AC (SHIP-1 inhibitor) inhibits platelet disaggregation, and promotes platelet ATP secretion. Upon ADP stimulation, Talin is recruited to αIIbβ3, and it is dissociated from αIIbβ3 when platelets disaggregate. In addition, treatment with RUC2, an inhibitor of αIIbβ3, which blocks αIIbβ3-mediated outside-in signalling, can markedly prevent the dissociation of talin from integrin. SHIP1 Inhibitor 3AC inhibits the dissociation of talin from integrin-β3. These results suggest that integrin-αIIbβ3-mediated outside-in signalling can serve as a brake to restrict unnecessary platelet activation by activated SHIP-1, which mediated the disassociation of talin from β3, leading to integrin inactivation and blocking of PI3K/Akt signalling to restrict platelet ATP secretion.

Role of Munc13-4 as a Ca2+-dependent tether during platelet secretion

The Munc13 family of exocytosis regulators has multiple Ca(2+)-binding, C2 domains. Here, we probed the mechanism by which Munc13-4 regulates in vitro membrane fusion and platelet exocytosis. We show that Munc13-4 enhances in vitro soluble NSF attachment protein receptor (SNARE)-dependent, proteoliposome fusion in a Ca(2+)- and phosphatidylserine (PS)-dependent manner that was independent of SNARE concentrations. Munc13-4-SNARE interactions, under the conditions used, were minimal in the absence or presence of Ca(2+). However, Munc13-4 was able to bind and cluster liposomes harbouring PS in response to Ca(2+). Interestingly, Ca(2+)-dependent liposome binding/clustering and enhancement of proteoliposome fusion required both Munc13-4 C2 domains, but only the Ca(2+)-liganding aspartate residues of the C2B domain. Analytical ultracentrifugation (AUC) measurements indicated that, in solution, Munc13-4 was a monomeric prolate ellipsoid with dimensions consistent with a molecule that could bridge two fusing membranes. To address the potential role of Munc13-4 as a tethering protein in platelets, we examined mepacrine-stained, dense granule mobility and secretion in platelets from wild-type and Munc13-4 null (Unc13d(Jinx)) mice. In the absence of Munc13-4, dense granules were highly mobile in both resting and stimulated platelets, and stimulation-dependent granule release was absent. These observations suggest that dense granules are stably docked in resting platelets awaiting stimulation and that Munc13-4 plays a vesicle-stabilizing or tethering role in resting platelets and also in activated platelets in response to Ca(2+). In summary, we show that Munc13-4 conveys Ca(2+) sensitivity to platelet SNARE-mediated membrane fusion and reveal a potential mechanism by which Munc13-4 bridges and stabilizes apposing membranes destined for fusion.

Platelet secretory behaviour: as diverse as the granules … or not?

Platelets play a central role in the arrest of bleeding after damage to a blood vessel and in the development of thrombosis. Platelets rapidly respond after interaction with sub-endothelial components and release cargo from their storage granules. The three principal granule types of platelets are α-granules, dense granules and lysosomes. Timed release of granule contents and regulated expression of critical receptors are essential for maintenance of the platelet thrombus, yet also have important functions beyond hemostasis (i.e. inflammatory reactions and immune responses). α-granules store adhesive molecules such as von Willebrand factor and fibrinogen, growth factors and inflammatory and angiogenic mediators, which play crucial roles in inflammatory responses and tumor genesis. The α-granules comprise a group of subcellular compartments with a unique composition and ultrastructure. Recent studies have suggested that differential secretory kinetics of α-granule subtypes is responsible for a thematic release of adhesive and inflammatory mediators. In addition, new results indicate that activation-dependent synthesis and release of cytokines also contribute to the inflammatory role of platelets. We will discuss the various methods that platelets use to regulate secretory processes and how these relate to potential differential secretion patterns, thereby promoting adhesiveness and/or inflammatory functions. We will focus on the heterogenic granule population, open canalicular system (OCS) plasticity, the role of contractile and mechanobiological forces, and the fusogenic machinery.

Identification of roles for the SNARE-associated protein, SNAP29, in mouse platelets

Platelets are critical for maintaining vascular hemostasis, but also play a major role in the formation of occlusive cardiovascular and cerebrovascular thrombi under disease conditions. Secretion of platelet alpha and dense granules is a requirement for efficient thrombus formation. Understanding and targeting the mechanisms of secretion is important to aid the development of effective antithrombotics. SNAP29 is a tSNARE found in platelets, but whose role has not been defined. Using a platelet-specific SNAP29 knockout mouse model, we assessed the role of SNAP29 in platelet secretion and function under standardized conditions and also in in vitro and in vivo thrombosis. The data showed no major defects in SNAP29-null platelets, but revealed a minor defect in α-granule secretion and a significant increase in embolization rate of thrombi in vivo. These data suggest that SNAP29 contributes to the regulation of platelet α-granule secretion and thrombus stability, possibly partially masked by functional redundancy with other tSNAREs, such as SNAP23.

Platelet secretion paves the way

In this issue of Blood, Sakurai et al examine the response of single platelets to fibrinogen- and collagen-coated microdots and show that platelets can orient their release of α-granule cargo to promote spreading beyond the dot's boundary.

TPC2 mediates new mechanisms of platelet dense granule membrane dynamics through regulation of Ca2+ release

TPC2 is a component of the platelet dense granule membrane and regulates the formation of perigranular Ca²⁺ nanodomains that correlate with “kiss-and-run” events and tubule connections. These membrane contacts allow material transfer between organelles and are likely involved in granule maturation.

Platelet dense granules (PDGs) are acidic calcium stores essential for normal hemostasis. They develop from late endosomal compartments upon receiving PDG-specific proteins through vesicular trafficking, but their maturation process is not well understood. Here we show that two-pore channel 2 (TPC2) is a component of the PDG membrane that regulates PDG luminal pH and the pool of releasable Ca²⁺. Using a genetically encoded Ca²⁺ biosensor and a pore mutant TPC2, we establish the function of TPC2 in Ca²⁺ release from PDGs and the formation of perigranular Ca²⁺ nanodomains. For the first time, Ca²⁺ spikes around PDGs—or any organelle of the endolysosome family—are visualized in real time and revealed to precisely mark organelle “kiss-and-run” events. Further, the presence of membranous tubules transiently connecting PDGs is revealed and shown to be dramatically enhanced by TPC2 in a mechanism that requires ion flux through TPC2. “Kiss-and-run” events and tubule connections mediate transfer of membrane proteins and luminal content between PDGs. The results show that PDGs use previously unknown mechanisms of membrane dynamics and content exchange that are regulated by TPC2.

Impact of the serum- and glucocorticoid-inducible kinase 1 on platelet dense granule biogenesis and secretion

BACKGROUND

Platelet secretion is critical to development of acute thrombotic occlusion. Platelet dense granules contain a variety of important hemostatically active substances. Nevertheless, biogenesis of platelet granules is poorly understood.

OBJECTIVES

Serum- and glucocorticoid-inducible kinase 1 (SGK1) has been shown to be highly expressed in platelets and megakaryocytes, but its role in the regulation of platelet granule biogenesis and its impact on thrombosis has not been investigated so far.

METHODS AND RESULTS

Electron microscopy analysis of the platelet ultrastructure revealed a significant reduction in the number and packing of dense granules in platelets lacking SGK1 (sgk1(-/-) ). In sgk1(-/-) platelets serotonin content was significantly reduced and activation-dependent secretion of ATP, serotonin and CD63 significantly impaired. In vivo adhesion after carotis ligation was significantly decreased in platelets lacking SGK1 and occlusive thrombus formation after FeCl3 -induced vascular injury was significantly diminished in sgk1(-/-) mice. Transcript levels and protein abundance of dense granule biogenesis regulating GTPase Rab27b were significantly reduced in sgk1(-/-) platelets without affecting Rab27b mRNA stability. In MEG-01 cells transfection with constitutively active (S422) (D) SGK1 but not with inactive (K127) (N) SGK1 significantly enhanced Rab27b mRNA levels. Sgk1(-/-) megakaryocytes show significantly reduced expression of Rab27b and serotonin/CD63 levels compared with sgk1(+/+) megakaryocytes. Proteome analysis identified nine further vesicular transport proteins regulated by SGK1, which may have an impact on impaired platelet granule biogenesis in sgk1(-/-) platelets independent of Rab27b.

CONCLUSIONS

The present observations identify SGK1 as a novel powerful regulator of platelet dense granule biogenesis, platelet secretion and thrombus formation. SGK1 is at least partially effective because it regulates transcription of Rab27b in megakaryocytes.

VAMP-7 links granule exocytosis to actin reorganization during platelet activation

Platelet activation results in profound morphologic changes accompanied by release of granule contents. Recent evidence indicates that fusion of granules with the plasma membrane during activation provides auxiliary membrane to cover growing actin structures. Yet little is known about how membrane fusion is coupled with actin reorganization. Vesicle-associated membrane protein (VAMP)-7 is found on platelet vesicles and possesses an N-terminal longin domain capable of linking exocytosis to cytoskeletal remodeling. We have evaluated platelets from VAMP-7(-/-) mice to determine whether this VAMP isoform contributes to granule release and platelet spreading. VAMP-7(-/-) platelets demonstrated a partial defect in dense granule exocytosis and impaired aggregation. α Granule exocytosis from VAMP-7(-/-) platelets was diminished both in vitro and in vivo during thrombus formation. Consistent with a role of VAMP-7 in cytoskeletal remodeling, spreading on matrices was decreased in VAMP-7(-/-) platelets compared to wild-type controls. Immunoprecipitation of VAMP-7 revealed an association with VPS9-domain ankyrin repeat protein (VARP), an adaptor protein that interacts with both membrane-bound and cytoskeleton proteins and with Arp2/3. VAMP-7, VARP, and Arp2/3 localized to the platelet periphery during spreading. These studies demonstrate that VAMP-7 participates in both platelet granule secretion and spreading and suggest a mechanism whereby VAMP-7 links granule exocytosis with actin reorganization.

Defective platelet autocrine signaling in HPS

In this issue of Blood, Meng et al and Sharda et al use the Hermansky-Pudlak syndrome (HPS) as a model to show that adenosine 5′-diphosphate (ADP) released by dense granules serves as an autocrine signal to potentiate platelet release of α-granule and lysosome cargo and protein disulfide isomerase (PDI), all of which serve to stabilize thrombus formation.

Defective PDI release from platelets and endothelial cells impairs thrombus formation in Hermansky-Pudlak syndrome

Protein disulfide isomerase (PDI), secreted from platelets and endothelial cells after injury, is required for thrombus formation. The effect of platelet and endothelial cell granule contents on PDI-mediated thrombus formation was studied by intravital microscopy using a mouse model of Hermansky-Pudlak syndrome in which platelet dense granules are absent. Platelet deposition and fibrin generation were nearly absent, and extracellular PDI was significantly reduced in HPS6(-/-) mice after vascular injury. HPS6(-/-) platelets displayed impaired PDI secretion and impaired exocytosis of α granules, lysosomes, and T granules due to decreased sensitivity to thrombin, but these defects could be corrected by addition of subthreshold amounts of adenosine 5'-diphosphate (ADP). Human Hermansky-Pudlak syndrome platelets demonstrated similar characteristics. Infusion of wild-type platelets rescued thrombus formation in HPS6(-/-) mice. Human umbilical vein endothelial cells in which the HPS6 gene was silenced displayed impaired PDI secretion and exocytosis of Weibel-Palade bodies. Defective thrombus formation in Hermansky-Pudlak syndrome, associated with impaired exocytosis of residual granules in endothelial cells and platelets, the latter due to deficiency of ADP, is characterized by a defect in T granule secretion, a deficiency in extracellular PDI secretion, and impaired fibrin generation and platelet aggregation. Hermansky-Pudlak syndrome is an example of a hereditary disease whereby impaired PDI secretion contributes to a bleeding phenotype.

Defective release of α granule and lysosome contents from platelets in mouse Hermansky-Pudlak syndrome models

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, bleeding diathesis, and other variable symptoms. The bleeding diathesis has been attributed to δ storage pool deficiency, reflecting the malformation of platelet dense granules. Here, we analyzed agonist-stimulated secretion from other storage granules in platelets from mouse HPS models that lack adaptor protein (AP)-3 or biogenesis of lysosome-related organelles complex (BLOC)-3 or BLOC-1. We show that α granule secretion elicited by low agonist doses is impaired in all 3 HPS models. High agonist doses or supplemental adenosine 5'-diphosphate (ADP) restored normal α granule secretion, suggesting that the impairment is secondary to absent dense granule content release. Intravital microscopy following laser-induced vascular injury showed that defective hemostatic thrombus formation in HPS mice largely reflected reduced total platelet accumulation and affirmed a reduced area of α granule secretion. Agonist-induced lysosome secretion ex vivo was also impaired in all 3 HPS models but was incompletely rescued by high agonist doses or excess ADP. Our results imply that (1) AP-3, BLOC-1, and BLOC-3 facilitate protein sorting to lysosomes to support ultimate secretion; (2) impaired secretion of α granules in HPS, and to some degree of lysosomes, is secondary to impaired dense granule secretion; and (3) diminished α granule and lysosome secretion might contribute to pathology in HPS.

Platelet secretion and hemostasis require syntaxin-binding protein STXBP5

Genome-wide association studies (GWAS) have linked genes encoding several soluble NSF attachment protein receptor (SNARE) regulators to cardiovascular disease risk factors. Because these regulatory proteins may directly affect platelet secretion, we used SNARE-containing complexes to affinity purify potential regulators from human platelet extracts. Syntaxin-binding protein 5 (STXBP5; also known as tomosyn-1) was identified by mass spectrometry, and its expression in isolated platelets was confirmed by RT-PCR analysis. Coimmunoprecipitation studies showed that STXBP5 interacts with core secretion machinery complexes, such as syntaxin-11/SNAP23 heterodimers, and fractionation studies suggested that STXBP5 also interacts with the platelet cytoskeleton. Platelets from Stxbp5 KO mice had normal expression of other key secretory components; however, stimulation-dependent secretion from each of the 3 granule types was markedly defective. Secretion defects in STXBP5-deficient platelets were confirmed via lumi-aggregometry and FACS analysis for P-selectin and LAMP-1 exposure. Interestingly, STXBP5-deficient platelets had altered granule cargo levels, despite having normal morphology and granule numbers. Consistent with secretion and cargo deficiencies, Stxbp5 KO mice showed dramatic bleeding in the tail transection model and defective hemostasis in the FeCl3-induced carotid injury model. Transplantation experiments indicated that these defects were due to loss of STXBP5 in BM-derived cells. Our data demonstrate that STXBP5 is required for normal arterial hemostasis, due to its contributions to platelet granule cargo packaging and secretion.

Secrets of platelet exocytosis - what do we really know about platelet secretion mechanisms?

Upon activation by extracellular matrix components or soluble agonists, platelets release in excess of 300 active molecules from intracellular granules. Those factors can both activate further platelets and mediate a range of responses in other cells. The complex microenvironment of a growing thrombus, as well as platelets' roles in both physiological and pathological processes, require platelet secretion to be highly spatially and temporally regulated to ensure appropriate responses to a range of stimuli. However, how this regulation is achieved remains incompletely understood. In this review we outline the importance of regulated secretion in thrombosis as well as in 'novel' scenarios beyond haemostasis and give a detailed summary of what is known about the molecular mechanisms of platelet exocytosis. We also discuss a number of theories of how different cargoes could be released in a tightly orchestrated manner, allowing complex interactions between platelets and their environment.

Advances in platelet granule biology

PURPOSE OF REVIEW

This review will provide an overview of several recent advances in the field of platelet granule biology.

RECENT FINDINGS

The past few years have witnessed a substantial evolution in our knowledge of platelet granules based on a number of discoveries and new experimental approaches. This article will cover recent studies in five areas. First, the vesicle trafficking pathways responsible for α-granule formation are beginning to be assembled as a result of the characterization of patients with α-granule deficiencies. Second, a revision of our understanding of which SNARE isoforms mediate platelet granule exocytosis has occurred following evaluation of patients with defects in platelet granule exocytosis and the generation of mice lacking specific SNAREs. Third, investigators have begun to establish how cargos are segregated among α-granules and determine whether or not different α-granule subpopulations exist in platelets. Fourth, an unanticipated role for α-granules in platelet spreading has been identified. Fifth, single-cell amperometry has revealed secretion kinetics with submillisecond temporal resolution enabling evaluation of the molecular control of the platelet fusion pore.

SUMMARY

These new observations reveal a previously unappreciated complexity to platelet granule formation and exocytosis and challenge our earlier notions of how these granules are organized within platelets and contribute to the multitude of physiological activities in which platelets function.

Abnormal megakaryocyte development and platelet function in Nbeal2(-/-) mice

Gray platelet syndrome (GPS) is an inherited bleeding disorder associated with macrothrombocytopenia and α-granule-deficient platelets. GPS has been linked to loss of function mutations in NEABL2 (neurobeachin-like 2), and we describe here a murine GPS model, the Nbeal2(-/-) mouse. As in GPS, Nbeal2(-/-) mice exhibit splenomegaly, macrothrombocytopenia, and a deficiency of platelet α-granules and their cargo, including von Willebrand factor (VWF), thrombospondin-1, and platelet factor 4. The platelet α-granule membrane protein P-selectin is expressed at 48% of wild-type levels and externalized upon platelet activation. The presence of P-selectin and normal levels of VPS33B and VPS16B in Nbeal2(-/-) platelets suggests that NBEAL2 acts independently of VPS33B/VPS16B at a later stage of α-granule biogenesis. Impaired Nbeal2(-/-) platelet function was shown by flow cytometry, platelet aggregometry, bleeding assays, and intravital imaging of laser-induced arterial thrombus formation. Microscopic analysis detected marked abnormalities in Nbeal2(-/-) bone marrow megakaryocytes, which when cultured showed delayed maturation, decreased survival, decreased ploidy, and developmental abnormalities, including abnormal extracellular distribution of VWF. Our results confirm that α-granule secretion plays a significant role in platelet function, and they also indicate that abnormal α-granule formation in Nbeal2(-/-) mice has deleterious effects on megakaryocyte survival, development, and platelet production.

IκB kinase phosphorylation of SNAP-23 controls platelet secretion

Platelet secretion plays a key role in thrombosis, thus the platelet secretory machinery offers a unique target to modulate hemostasis. We report the regulation of platelet secretion via phosphorylation of SNAP-23 at Ser95. Phosphorylation of this t-soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) occurs upon activation of known elements of the platelet signaling cascades (ie, phospholipase C, [Ca(2+)]i, protein kinase C) and requires IκB kinase (IKK)-β. Other elements of the nuclear factor κB/IκB cascade (ie, IKK-α,-β,-γ/NEMO and CARMA/MALT1/Bcl10 complex) are present in anucleate platelets and IκB is phosphorylated upon activation, suggesting that this pathway is active in platelets and implying a nongenomic role for IKK. Inhibition of IKK-β, either pharmacologically (with BMS-345541, BAY11-7082, or TPCA-1) or by genetic manipulation (platelet factor 4 Cre:IKK-β(flox/flox)), blocked SNAP-23 phosphorylation, platelet secretion, and SNARE complex formation; but, had no effect on platelet morphology or other metrics of platelet activation. Consistently, SNAP-23 phosphorylation enhanced membrane fusion of SNARE-containing proteoliposomes. In vivo studies with IKK inhibitors or platelet-specific IKK-β knockout mice showed that blocking IKK-β activity significantly prolonged tail bleeding times, suggesting that currently available IKK inhibitors may affect hemostasis.

Chorein sensitivity of cytoskeletal organization and degranulation of platelets

Chorea-acanthocytosis (ChAc), a lethal disease caused by defective chorein, is characterized by neurodegeneration and erythrocyte acanthocytosis. The functional significance of chorein in other cell types remained ill-defined. The present study revealed chorein expression in blood platelets. As compared to platelets from healthy volunteers, platelets from patients with ChAc displayed a 47% increased globular/filamentous actin ratio, indicating actin depolymerization. Moreover, phosphoinositide-3-kinase subunit p85 phosphorylation, p21 protein-activated kinase (PAK1) phosphorylation, as well as vesicle-associated membrane protein 8 (VAMP8) expression were significantly reduced in platelets from patients with ChAc (by 17, 22, and 39%, respectively) and in megakaryocytic (MEG-01) cells following chorein silencing (by 16, 54, and 11%, respectively). Activation-induced platelet secretion from dense granules (ATP release) and α granules (P-selectin exposure) were significantly less (by 55% after stimulation with 1 μg/ml CRP and by 33% after stimulation with 5 μM TRAP, respectively) in ChAc platelets than in control platelets. Furthermore, platelet aggregation following stimulation with different platelet agonists was significantly impaired. These observations reveal a completely novel function of chorein, i.e., regulation of secretion and aggregation of blood platelets.

Dynamin-related protein-1 controls fusion pore dynamics during platelet granule exocytosis

OBJECTIVE

Platelet granule exocytosis serves a central role in hemostasis and thrombosis. Recently, single-cell amperometry has shown that platelet membrane fusion during granule exocytosis results in the formation of a fusion pore that subsequently expands to enable the extrusion of granule contents. However, the molecular mechanisms that control platelet fusion pore expansion and collapse are not known.

METHODS AND RESULTS

We identified dynamin-related protein-1 (Drp1) in platelets and found that an inhibitor of Drp1, mdivi-1, blocked exocytosis of both platelet dense and α-granules. We used single-cell amperometry to monitor serotonin release from individual dense granules and, thereby, measured the effect of Drp1 inhibition on fusion pore dynamics. Inhibition of Drp1 increased spike width and decreased prespike foot events, indicating that Drp1 influences fusion pore formation and expansion. Platelet-mediated thrombus formation in vivo after laser-induced injury of mouse cremaster arterioles was impaired after infusion of mdivi-1.

CONCLUSIONS

These results demonstrate that inhibition of Drp1 disrupts platelet fusion pore dynamics and indicate that Drp1 can be targeted to control thrombus formation in vivo.

Platelet granule exocytosis: a comparison with chromaffin cells

The rapid secretion of bioactive amines from chromaffin cells constitutes an important component of the fight or flight response of mammals to stress. Platelets respond to stresses within the vasculature by rapidly secreting cargo at sites of injury, inflammation, or infection. Although chromaffin cells derive from the neural crest and platelets from bone marrow megakaryocytes, both have evolved a heterogeneous assemblage of granule types and a mechanism for efficient release. This article will provide an overview of granule formation and exocytosis in platelets with an emphasis on areas in which the study of chromaffin cells has influenced that of platelets and on similarities between the two secretory systems. Commonalities include the use of transporters to concentrate bioactive amines and other cargos into granules, the role of cytoskeletal remodeling in granule exocytosis, and the use of granules to provide membrane for cytoplasmic projections. The SNAREs and SNARE accessory proteins used by each cell type will also be considered. Finally, we will discuss the newly appreciated role of dynamin family proteins in regulated fusion pore formation. This evaluation of the comparative cell biology of regulated exocytosis in platelets and chromaffin cells demonstrates a convergence of mechanisms between two disparate cell types both tasked with responding rapidly to physiological stimuli.

Mechanism of platelet dense granule biogenesis: study of cargo transport and function of Rab32 and Rab38 in a model system

Dense granules are important in platelet aggregation to form a hemostatic plug as evidenced by the increased bleeding time in mice and humans with dense granule deficiency. Dense granules also are targeted by antiplatelet agents because of their role in thrombus formation. Therefore, the molecular understanding of the dense granule and its biogenesis is of vital importance. In this work, we establish a human megakaryocytic cell line (MEG-01) as a model system for the study of dense granule biogenesis using a variety of cell biology and biochemical approaches. Using this model system, we determine the late endocytic origin of these organelles by colocalization of the internalized fluid phase marker dextran with both mepacrine and transmembrane dense granule proteins. By mistargeting of mutant dense granule proteins, we demonstrate that sorting signals recognized by adaptor protein-3 are necessary for normal transport to dense granules. Furthermore, we show that tissue-specific Rab32 and Rab38 are crucial for the fusion of vesicles containing dense granule cargo with the maturing organelle. This work sheds light on the biogenesis of dense granules at the molecular level and opens the possibility of using this powerful model system for the investigation of new components of the biogenesis machinery.