Human Platelets Contain SNARE Proteins and a Sec1p Homologue That Interacts With Syntaxin 4 and Is Phosphorylated After Thrombin Activation: Implications for Platelet Secretion

Syntaxin-1a and SNAP-25 expression level is increased in the blood samples of ischemic stroke patients

The interest for the discovery of blood biomarkers for several neurological disorders, including Ischemic Stroke (IS), is growing and their identification in blood samples would be revolutionary allowing a fast and better pathology prediction or outcome and to collect information on patient recovery. The increased permeability of the blood-brain barrier, following a brain infarct, allows the detection of brain proteins in the blood flow. In this work, we analyzed the expression levels of two synaptic proteins Syntaxin (STX)-1a and Synaptosomal Associated Protein, 25 kDa (SNAP-25), in Peripheral Blood Mononuclear Cell (PBMC), serum and in Neuronal Derived Extracellular vesicles (NDEs) of IS patients, age and sex matched healthy control (HC) and younger HC (Y-HC). Interestingly, we identified STX-1a protein in the cytoplasm of PBMC and both STX-1a and SNAP-25 expression levels were significantly augmented in all IS patient's blood fractions compared to control subjects. In addition, STX-1a blood levels correlated with the IS clinical scales National Institutes of Health Stroke Scale (NIH-SS) and the modified Barthel Index (BI). These results prompted us to speculate that STX-1a and SNAP-25 hematic fluctuations depict the brain damage after an ischemic attack and that their hematic detection could represent a novel and accessible IS biomarkers.

Dynamic Growth and Shrinkage of the Salmonella-Containing Vacuole Determines the Intracellular Pathogen Niche

Salmonella is a human and animal pathogen that causes gastro-enteric diseases. The key to Salmonella infection is its entry into intestinal epithelial cells, where the bacterium resides within a Salmonella-containing vacuole (SCV). Salmonella entry also induces the formation of empty macropinosomes, distinct from the SCV, in the vicinity of the entering bacteria. A few minutes after its formation, the SCV increases in size through fusions with the surrounding macropinosomes. Salmonella also induces membrane tubules that emanate from the SCV and lead to SCV shrinkage. Here, we show that these antipodal events are utilized by Salmonella to either establish a vacuolar niche or to be released into the cytosol by SCV rupture. We identify the molecular machinery underlying dynamic SCV growth and shrinkage. In particular, the SNARE proteins SNAP25 and STX4 participate in SCV inflation by fusion with macropinosomes. Thus, host compartment size control emerges as a pathogen strategy for intracellular niche regulation.

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The early SCV simultaneously grows and shrinks through fusion and tubule formation

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SCV shrinkage promotes vacuolar rupture and cytosolic release

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IAMs are enriched in the host SNAREs SNAP25 and STX4, enabling IAM-SCV fusion

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Promoting SNX1-mediated tubule formation, SopB fosters SCV ruptures

Vitamin D deficiency induces the excitation/inhibition brain imbalance and the proinflammatory shift

Vitamin D₃ is among the major neurosteroids whose role in developing and adult brain is intensively studied now. Its active form 1,25(OH)₂D₃ regulates the expression and functioning of a range of brain-specific proteins, which orchestrate the neurotransmitter turnover, neurogenesis and neuroplasticity. Despite numerous studies of the vitamin D role in normal and pathological brain function, there is little evidence on the mechanisms of alterations in excitatory and inhibitory neurotransmission under vitamin D deficiency (VDD). Using the animal model we characterized the dysfunction of excitatory and inhibitory neurotransmission under alimentary VDD. The shift between unstimulated and evoked GABA release under VDD was largely reversed after treatment of VDD, whereas the impairments in glutamatergic system were only partially recovered after 1-month vitamin D₃ supplementation. The increase of the external glutamate level and unstimulated GABA release in brain nerve terminals was associated with intensified ROS production and higher [Ca²⁺]_i in presynapse. The negative allosteric modulation of presynaptic mGlu7 receptors significantly enhanced exocytotic GABA release, which was decreased under VDD, thereby suggesting the neuroprotective effect of such modulation of inhibitory neurotransmission. Synaptic plasma membranes and cytosolic proteins contribute to the decreased stimulated release of neurotransmitter, by being the crucial components, whose functional state is impaired under VDD. The critical changes with synaptic vesicles occurred at the docking step of the process, whereas malfunctioning of synaptic cytosolic proteins impacted the fusion event foremost. The decreased amplitude of exocytosis was inherent for non-excitable cells as well, as evidenced by lower platelet degranulation. Our data suggest the presynaptic dysfunction and proinflammatory shift as the early events in the pathogenesis of VDD-associated disorders and provide evidences for the neuroprotective role of vitamin D₃.

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.

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.

Diverse exocytic pathways for mast cell mediators

Mast cells play pivotal roles in innate and adaptive immunities but are also culprits in allergy, autoimmunity, and cardiovascular diseases. Mast cells respond to environmental changes by initiating regulated exocytosis/secretion of various biologically active compounds called mediators (e.g. proteases, amines, and cytokines). Many of these mediators are stored in granules/lysosomes and rely on intricate degranulation processes for release. Mast cell stabilizers (e.g. sodium cromoglicate), which prevent such degranulation processes, have therefore been clinically employed to treat asthma and allergic rhinitis. However, it has become increasingly clear that different mast cell diseases often involve multiple mediators that rely on overlapping but distinct mechanisms for release. This review illustrates existing evidence that highlights the diverse exocytic pathways in mast cells. We also discuss strategies to delineate these pathways so as to identify unique molecular components which could serve as new drug targets for more effective and specific treatments against mast cell-related diseases.

Mammalian Septin Function in Hemostasis and Beyond

Interest in the biology of mammalian septin proteins has undergone a birth in recent years. Originally identified as critical for yeast budding throughout the 1970s, the septin family is now recognized to extend from yeast to humans and is associated with a variety of events ranging from cytokinesis to vesicle trafficking. An emerging theme for septins is their presence at sites where active membrane or cytoplasmic partitioning is occurring. Here, we briefly review the mammalian septin protein family and focus on a prototypic human and mouse septin, termed SEPT5, that is expressed in the brain, heart, and megakaryocytes. Work from neurobiology laboratories has linked SEPT5 to the exocytic complex of neurons, with implications that SEPT5 regulates neurotransmitter release. Striking similarities exist between neurotransmitter release and the platelet-release reaction, which is a critical step in platelet response to vascular injury. Work from our laboratory has characterized the platelet phenotype from mice containing a targeted deletion of SEPT5. Most strikingly, platelets from SEPT5null animals aggregate and release granular contents in response to subthreshold levels of agonists. Thus, the characterization of a SEPT5-deficient mouse has linked SEPT5 to the Platelet exocytic process and, as such, illustrates it as an important protein for regulating platelet function. Recent data suggest that platelets contain a wide repertoire of different septin proteins and assemble to form macromolecular septin complexes. The mouse platelet provides an experimental framework to define septin function in hemostasis, with implications for neurobiology and beyond.

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.

A unique SNARE machinery for exocytosis of cytotoxic granules and platelets granules

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells target infected or transformed cells with perforin-containing cytotoxic granules through immune synapses, while platelets secrete several types of granules which contents are essential for thrombosis and hemostasis. Recent work has culminated in the notion that an exocytic SNARE complex, based on a very similar set of components, is primarily responsible for exocytosis of the diverse granules in these different cell types. Granule exocytosis is, in particular, uniquely dependent on the atypical Q-SNARE syntaxin 11, its interacting partners of the Sec/Munc (SM) family, and is regulated by Rab27a. Mutations in these exocytic components underlie disease manifestations of familial hemophagocytic lymphohistiocytosis (FHL) subtypes, characterized by hyperactivation of the immune system, as well as platelet granule secretion defects. Here we discuss the key discoveries that led to the converging notion of the syntaxin 11-based exocytosis machinery for cytotoxic granules and platelet-derived granules.

NF-κB signaling pathway as target for antiplatelet activity

In different nucleated cells, NF-κB has long been considered a prototypical proinflammatory signaling pathway with the expression of proinflammatory genes. Although platelets lack a nucleus, a number of functional transcription factors are involved in activated platelets, such as NF-κB. In platelet activation NF-κB regulation events include IKKβ phosphorylation, IκBα degradation, and p65 phosphorylation. Multiple pathways contribute to platelet activation and NF-κB is a common pathway in this activation. Therefore, in platelet activation the modulation of NF-κB pathway could be a potential new target in the treatment of inflammation-related vascular disease therapy (antiplatelet and antithrombotic activities).

Platelet signaling: a complex interplay between inhibitory and activatory networks

The role of platelets in hemostasis and thrombosis is dependent on a complex balance of activatory and inhibitory signaling pathways. Inhibitory signals released from the healthy vasculature suppress platelet activation in the absence of platelet receptor agonists. Activatory signals present at a site of injury initiate platelet activation and thrombus formation; subsequently, endogenous negative signaling regulators dampen activatory signals to control thrombus growth. Understanding the complex interplay between activatory and inhibitory signaling networks is an emerging challenge in the study of platelet biology, and necessitates a systematic approach to utilize experimental data effectively. In this review, we will explore the key points of platelet regulation and signaling that maintain platelets in a resting state, mediate activation to elicit thrombus formation, or provide negative feedback. Platelet signaling will be described in terms of key signaling molecules that are common to the pathways activated by platelet agonists and can be described as regulatory nodes for both positive and negative regulators.

Cyclin-dependent kinase 5 regulates degranulation in human eosinophils

Degranulation from eosinophils in response to secretagogue stimulation is a regulated process that involves exocytosis of granule proteins through specific signalling pathways. One potential pathway is dependent on cyclin-dependent kinase 5 (Cdk5) and its effector molecules, p35 and p39, which play a central role in neuronal cell exocytosis by phosphorylating Munc18, a regulator of SNARE binding. Emerging evidence suggests a role for Cdk5 in exocytosis in immune cells, although its role in eosinophils is not known. We sought to examine the expression of Cdk5 and its activators in human eosinophils, and to assess the role of Cdk5 in eosinophil degranulation. We used freshly isolated human eosinophils and analysed the expression of Cdk5, p35, p39 and Munc18c by Western blot, RT-PCR, flow cytometry and immunoprecipitation. Cdk5 kinase activity was determined following eosinophil activation. Cdk5 inhibitors were used (roscovitine, AT7519 and small interfering RNA) to determine its role in eosinophil peroxidase (EPX) secretion. Cdk5 was expressed in association with Munc18c, p35 and p39, and phosphorylated following human eosinophil activation with eotaxin/CCL11, platelet-activating factor, and secretory IgA-Sepharose. Cdk5 inhibitors (roscovitine, AT7519) reduced EPX release when cells were stimulated by PMA or secretory IgA. In assays using small interfering RNA knock-down of Cdk5 expression in human eosinophils, we observed inhibition of EPX release. Our findings suggest that in activated eosinophils, Cdk5 is phosphorylated and binds to Munc18c, resulting in Munc18c release from syntaxin-4, allowing SNARE binding and vesicle fusion, with subsequent eosinophil degranulation. Our work identifies a novel role for Cdk5 in eosinophil mediator release by agonist-induced degranulation.

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.

Inhibitory effects of a cationic liposome on allergic reaction mediated by mast cell activation

Several studies have shown that cationic liposomes exert immunomodulatory effects with low immunogenicity and toxicity, and offer advantages such as easy preparation and targeting. Cationic liposomes not only transport DNA to immune cells but also enhance the function of antigen presenting cells such as dendritic cells and macrophages. Here, we investigated the effect of a particular cationic liposome on mast cell function during allergic reaction. We found that the cationic liposomes bound to the mast cell surface suppressed degranulation induced by cross-linking of high affinity immunoglobulin E receptors in a time- and dose-dependent manner. The suppression of degranulation was mediated by impairment of the sustained level of intracellular Ca(2+) concentration ([Ca(2+)]i) derived from the inhibition of store-operated Ca(2+) entry. The decrease in sustained elevation of [Ca(2+)]i led to the suppression of phosphorylation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins such as SNAP-23, syntaxin-4, which are necessary for membrane fusion between secretory granules and the plasma membrane during degranulation. Furthermore, the cationic liposomes suppressed vascular permeability elevation induced by mast cell activation in mice. These results showed that cationic liposomes possess the novel property of inhibiting mast cell activation, suggesting the possibility of developing cationic liposomes as anti-allergic effectors.

Munc18b/STXBP2 is required for platelet secretion

Platelets are vital for hemostasis because they release their granule contents in response to vascular damage. Platelet exocytosis is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), whose interactions are governed by regulators, eg, Sec/Munc18 proteins. These proteins chaperone syntaxin t-SNAREs and are required for exocytosis. Platelets contain 3 Munc18 isoforms: Munc18a, Munc18b, and Munc18c. We report that Munc18b is the major isoform and is required for platelet secretion. Familial hemophagocytic lymphohistiocytosis type 5 (FHL5) is caused by defects in the Munc18b/STXBP2 gene. We confirm a previous report showing that platelets from FHL5 patients have defective secretion. Serotonin, ADP/ATP, and platelet factor 4 release was profoundly affected in the 2 biallelic patients and partially in a heterozygous patient. Release of lysosomal contents was only affected in the biallelic platelets. Platelets from the FHL5 biallelic patients showed decreased Munc18b and syntaxin-11 levels were significantly reduced; other syntaxins were unaffected. Munc18b formed complexes with syntaxin-11, SNAP-23, and vesicle-associated membrane protein-8 in human platelets. Other potential secretion regulators, Munc13-4 and Rab27, were also found associated. These data demonstrate a key role for Munc18b, perhaps as a limiting factor, in platelet exocytosis and suggest that it regulates syntaxin-11.

Syntaxin-11, but not syntaxin-2 or syntaxin-4, is required for platelet secretion

The platelet release reaction plays a critical role in thrombosis and contributes to the events that follow hemostasis. Previous studies have shown that platelet secretion is mediated by Soluble NSF Attachment Protein Receptor (SNARE) proteins from granule and plasma membranes. The SNAREs form transmembrane complexes that mediate membrane fusion and granule cargo release. Although VAMP-8 (v-SNARE) and SNAP-23 (a t-SNARE class) are important for platelet secretion, the identity of the functional syntaxin (another t-SNARE class) has been controversial. Previous studies using anti-syntaxin Abs in permeabilized platelets have suggested roles for both syntaxin-2 and syntaxin-4. In the present study, we tested these conclusions using platelets from syntaxin-knockout mouse strains and from a Familial Hemophagocytic Lymphohistiocytosis type 4 (FHL4) patient. Platelets from syntaxin-2 and syntaxin-4 single- or double-knockout mice had no secretion defect. Platelets from a FHL4 patient deficient in syntaxin-11 had a robust defect in agonist-induced secretion although their morphology, activation, and cargo levels appeared normal. Semiquantitative Western blotting showed that syntaxin-11 is the more abundant syntaxin in both human and murine platelets. Coimmunoprecipitation experiments showed that syntaxin-11 can form SNARE complexes with both VAMP-8 and SNAP-23. The results of the present study indicate that syntaxin-11, but not syntaxin-2 or syntaxin-4, is required for platelet exocytosis.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Submaximal inhibition of protein kinase C restores ADP-induced dense granule secretion in platelets in the presence of Ca2+

Protein kinase C (PKC) is a family of serine/threonine kinases that play isoform-specific inhibitory and stimulatory roles in platelet activation. We show here that the pan-PKC inhibitor Ro31-8220 can be used to dissect these events following platelet activation by ADP. Submaximal concentrations of Ro31-8220 potentiated aggregation and dense granule secretion to ADP in plasma anticoagulated with citrate, in d-Phe-Pro-Arg-chloromethyl ketone-anticoagulated plasma, which has physiological levels of Ca²⁺, and in washed platelets. Potentiation was retained on inhibition of cyclooxygenase and was associated with an increase in intracellular Ca²⁺. Potentiation of aggregation and secretion was abolished by a maximally effective concentration of Ro31-8220, consistent with a critical role of PKC in secretion. ADP-induced secretion was potentiated in the presence of an inhibitor of PKCβ but not in the presence of available inhibitors of other PKC isoforms in human and mouse platelets. ADP-induced secretion was also potentiated in mouse platelets deficient in PKCϵ but not PKCθ. These results demonstrate that partial blockade of PKC potentiates aggregation and dense granule secretion by ADP in association with increased Ca²⁺. This provides a molecular explanation for the inability of ADP to induce secretion in plasma in the presence of physiological Ca²⁺ concentrations, and it reveals a novel role for PKC in inhibiting platelet activation by ADP in vivo. These results also demonstrate isoform-specific inhibitory effects of PKC in platelets.

The mechanisms of exocytosis in mast cells

Upon activation through high affinity IgE receptors (FcεRI), mast cells (MCs) can release up to 100% of their content of preformed mediators stored in cytoplasmic secretory granules by compound exocytosis. This causes Type I immediate hypersensitivity reactions and, in the case of inappropriate activation by allergens, the symptoms of allergy. Recent work has uncovered a central role of SNARE (Soluble N-ethylmaleimide-Sensitive Factor (NSF) Attachment Protein (SNAP) Receptors) proteins in regulating the numerous membrane fusion events during exocytosis. This has defined a series of new molecular actors in MC exocytosis that participate in the regulation of membrane fusion and the connection of the fusion machinery with early signaling events. The purpose of this chapter is to describe these proteins and provide a brief overview on their mechanism of action.

Fusogenic pairings of vesicle-associated membrane proteins (VAMPs) and plasma membrane t-SNAREs--VAMP5 as the exception

Background

Intracellular vesicle fusion is mediated by the interactions of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins on vesicles (v-SNAREs) and on target membranes (t-SNAREs). The vesicle-associated membrane proteins (VAMPs) are v-SNAREs that reside in various post-Golgi vesicular compartments. To fully understand the specific role of each VAMP in vesicle trafficking, it is important to determine if VAMPs have differential membrane fusion activities.

Methodology/Principal Findings

In this study, we developed a cell fusion assay that quantifies SNARE-mediated membrane fusion events by activated expression of β-galactosidase, and examined fusogenic pairings between the seven VAMPs, i.e., VAMPs 1, 2, 3, 4, 5, 7 and 8, and two plasma membrane t-SNARE complexes, syntaxin1/SNAP-25 and syntaxin4/SNAP-25. VAMPs 1, 2, 3, 4, 7 and 8 drove fusion efficiently, whereas VAMP5 was unable to mediate fusion with the t-SNAREs. By expressing VAMPs 1, 3, 4, 7 and 8 at the same level, we further compared their membrane fusion activities. VAMPs 1 and 3 had comparable and the highest fusion activities, whereas VAMPs 4, 7 and 8 exhibited 30–50% lower fusion activities. Moreover, we determined the dependence of cell fusion activity on VAMP1 expression level. Analysis of the dependence data suggested that there was no cooperativity of VAMP proteins in the cell fusion reaction.

Conclusions/Significance

These data indicate that VAMPs have differential membrane fusion capacities, and imply that with the exception of VAMP5, VAMPs are essentially redundant in mediating fusion with plasma membrane t-SNAREs.

Stimulus-secretion coupling by high-affinity IgE receptor: new developments

Stimulation of mast cells through their high affinity IgE receptor (FcεRI) leads to the secretion of pre- and neoformed soluble mediators through vesicular carriers. This process is highly regulated in order to adapt the secretion of these potentially dangerous factors to the physiological needs. This regulation requires numerous essential effectors that are necessary to transmit the initial signal of FcεRI aggregation and couple it to the sophisticated secretory machinery of membrane fusion. Studies in recent years have led to the discovery of a series of new effector molecules that link FcεRI to secretion. We describe here some of the new developments that have allowed to obtain a clearer picture of stimulus/secretion coupling in mast cells.

Munc13-4 is a limiting factor in the pathway required for platelet granule release and hemostasis

Activation-dependent platelet granule release is mediated by integral membrane proteins called soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) and their regulators; however, the mechanisms for this process are ill-defined. To further characterize platelet secretion, we analyzed the function of platelets from Unc13d(Jinx) mice. Platelets from these animals lack the putative vesicle priming factor, Munc13-4, and have a severe secretion defect. Release from dense granules was completely ablated and that from alpha-granules and lysosomes was severely compromised. Unc13d(Jinx) platelets showed attenuated aggregation and, consequently, Unc13d(Jinx) mice had prolonged tail-bleeding times. The secretion defect was not due to altered expression of SNAREs or SNARE regulators, defective granule biogenesis, or faulty platelet activation. The defective release could be rescued by adding recombinant Munc13-4 to permeabilized Unc13d(Jinx) platelets. In wild-type mouse platelets, Munc13-4 levels were lower than those of SNAREs suggesting that Munc13-4 could be a limiting component of the platelets' secretory machinery. Consistently, Munc13-4 levels directly correlated with the extent of granule release from permeabilized platelets and from intact, heterozygous Unc13d(Jinx) platelets. These data highlight the importance of Munc13-4 in platelets and indicate that it is a limiting factor required for platelet secretion and hemostasis.

Loss of pleckstrin defines a novel pathway for PKC-mediated exocytosis

Pleckstrin, the platelet and leukocyte C kinase substrate, is a prominent substrate of PKC in platelets, monocytes, macrophages, lymphocytes, and granulocytes. Pleckstrin accounts for 1% of the total protein in these cells, but it is best known for containing the 2 prototypic Pleckstrin homology, or PH, domains. Overexpressed pleckstrin can affect polyphosphoinositide second messenger-based signaling events; however, its true in vivo role has been unknown. Here, we describe mice containing a null mutation within the pleckstrin gene. Platelets lacking pleckstrin exhibit a marked defect in exocytosis of delta and alpha granules, alphaIIbbeta3 activation, actin assembly, and aggregation after exposure to the PKC stimulant, PMA. Pleckstrin-null platelets aggregate normally in response to thrombin, but they fail to aggregate in response to thrombin in the presence of PI3K inhibitors, suggesting that a PI3K-dependent signaling pathway compensates for the loss of pleckstrin. Although pleckstrin-null platelets merged their granules in response to stimulation of PKC, they failed to empty their contents into the open canalicular system. This might be attributable to impaired actin assembly present in cells lacking pleckstrin. These data show that pleckstrin regulates the fusion of granules to the cell membrane and is an essential component of PKC-mediated exocytosis.

PKCalpha regulates platelet granule secretion and thrombus formation in mice

Platelets are central players in atherothrombosis development in coronary artery disease. The PKC family provides important intracellular mechanisms for regulating platelet activity, and platelets express several members of this family, including the classical isoforms PKCalpha and PKCbeta and novel isoforms PKCdelta and PKCtheta. Here, we used a genetic approach to definitively demonstrate the role played by PKCalpha in regulating thrombus formation and platelet function. Thrombus formation in vivo was attenuated in Prkca-/- mice, and PKCalpha was required for thrombus formation in vitro, although this PKC isoform did not regulate platelet adhesion to collagen. The ablation of in vitro thrombus formation in Prkca-/- platelets was rescued by the addition of ADP, consistent with the key mechanistic finding that dense-granule biogenesis and secretion depend upon PKCalpha expression. Furthermore, defective platelet aggregation in response to either collagen-related peptide or thrombin could be overcome by an increase in agonist concentration. Evidence of overt bleeding, including gastrointestinal and tail bleeding, was not seen in Prkca-/- mice. In summary, the effects of PKCalpha ablation on thrombus formation and granule secretion may implicate PKCalpha as a drug target for antithrombotic therapy.

AQP2 exocytosis in the renal collecting duct -- involvement of SNARE isoforms and the regulatory role of Munc18b

Vasopressin regulates the fusion of the water channel aquaporin 2 (AQP2) to the apical membrane of the renal collecting-duct principal cells and several lines of evidence indicate that SNARE proteins mediate this process. In this work MCD4 renal cells were used to investigate the functional role of a set of Q- and R-SNAREs, together with that of Munc18b as a negative regulator of the formation of the SNARE complex. Both VAMP2 and VAMP3 were associated with immunoisolated AQP2 vesicles, whereas syntaxin 3 (Stx3), SNAP23 and Munc18 were associated with the apical plasma membrane. Co-immunoprecipitation experiments indicated that Stx3 forms complexes with VAMP2, VAMP3, SNAP23 and Munc18b. Protein knockdown coupled to apical surface biotinylation demonstrated that reduced levels of the R-SNAREs VAMP2 and VAMP3, and the Q-SNAREs Stx3 and SNAP23 strongly inhibited AQP2 fusion at the apical membrane. In addition, knockdown of Munc18b promoted a sevenfold increase of AQP2 fused at the plasma membrane without forskolin stimulation. Taken together these findings propose VAMP2, VAMP3, Stx3 and SNAP23 as the complementary set of SNAREs responsible for AQP2-vesicle fusion into the apical membrane, and Munc18b as a negative regulator of SNARE-complex formation in renal collecting-duct principal cells.

Corin is co-expressed with pro-ANP and localized on the cardiomyocyte surface in both zymogen and catalytically active forms

The multi-domain transmembrane serine protease corin cleaves pro-atrial natriuretic peptide (pro-ANP) in vitro to generate an active hormone, ANP. Corin may also contribute to the regulation of the natriuretic peptide system in vivo, and might be an attractive target for treatment of cardiovascular diseases. In order for corin to cleave its substrate pro-ANP, it should be catalytically active and located proximally. However, because knowledge of native corin is limited, we examined the expression, cardiac localization and molecular forms of the native corin protein. Immunofluorescence studies using a series of anti-corin antibodies directed against the stem and protease domains reveal that corin is present on the cell-surface of rat neonatal cardiomyocytes and murine HL-1 cardiomyocyte-like cells. Furthermore, we immunolocalized native corin in pro-ANP expressing cardiomyocytes. Immunoprecipitation of the membrane fraction of mouse heart extract showed that native corin had a relative mass of 205-210 kDa. Under reducing conditions native corin migrates as several different molecular weight forms corresponding to zymogen (uncleaved) and active (cleaved) forms. Studies using a FITC-tagged chloromethyl ketone that mimics the corin cleavage sequence in pro-ANP, suggest that an enzymatically active form of corin is localized to the cell surface of myocardial cells in vivo. Additionally, we showed that the 205-210 kDa form of corin is a glycosylated protein. Treatment of HL-1 cells with tunicamycin reduced the relative mass of expressed corin. We conclude that native corin is a glycosylated protease that is localized on the cell surface of pro-ANP-expressing cardiomyocytes in both zymogen and catalytically active forms.

Involvement of SNARE proteins in thrombin-induced platelet aggregation: Evidence for the relevance of Ca2+ entry

Thrombin induces platelet activation through a variety of intracellular mechanisms, including Ca(2+) mobilization. The protein of the exocytotic machinery SNAP-25, but not VAMPs, is required for store-operated Ca(2+) entry, the main mechanism for Ca(2+) influx in platelets. Hence, we have investigated the role of the SNAP-25 and VAMPs in thrombin-induced platelet aggregation. Platelet stimulation with thrombin or selective activation of thrombin receptors PAR-1, PAR-4 or GPIb-IX-V results in platelet aggregation that, except for GPIb-IX-V receptor, requires Ca(2+) entry for full activation. Depletion of the intracellular Ca(2+) stores using pharmacological tools was unable to induce aggregation except when cytosolic Ca(2+) concentration reached a critical level (around 1.5 microM). Electrotransjection of cells with anti-SNAP-25 antibody reduced thrombin-evoked platelet aggregation, while electrotransjection of anti-VAMP-1, -2 and -3 antibody had no effect. These findings support a role for SNAP-25 but not VAMP-1, -2 and -3 in platelet aggregation, which is likely mediated by the regulation of Ca(2+) mobilization in human platelets.

Membrane fusion by VAMP3 and plasma membrane t-SNAREs

Pairing of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins on vesicles (v-SNAREs) and SNARE proteins on target membranes (t-SNAREs) mediates intracellular membrane fusion. VAMP3/cellubrevin is a v-SNARE that resides in recycling endosomes and endosome-derived transport vesicles. VAMP3 has been implicated in recycling of transferrin receptors, secretion of alpha-granules in platelets, and membrane trafficking during cell migration. Using a cell fusion assay, we examined membrane fusion capacity of the ternary complexes formed by VAMP3 and plasma membrane t-SNAREs syntaxin1, syntaxin4, SNAP-23 and SNAP-25. VAMP3 forms fusogenic pairing with t-SNARE complexes syntaxin1/SNAP-25, syntaxin1/SNAP-23 and syntaxin4/SNAP-25, but not with syntaxin4/SNAP-23. Deletion of the N-terminal domain of syntaxin4 enhanced membrane fusion more than two fold, indicating that the N-terminal domain negatively regulates membrane fusion. Differential membrane fusion capacities of the ternary v-/t-SNARE complexes suggest that transport vesicles containing VAMP3 have distinct membrane fusion kinetics with domains of the plasma membrane that present different t-SNARE proteins.

SNAP-23 and syntaxin-2 localize to the extracellular surface of the platelet plasma membrane

SNARE proteins direct membrane fusion events required for platelet granule secretion. These proteins are oriented in cell membranes such that most of the protein resides in a cytosolic compartment. Evaluation of SNARE protein localization in activated platelets using immunonanogold staining and electron microscopy, however, demonstrated expression of SNAP-23 and syntaxin-2 on the extracellular surface of the platelet plasma membrane. Flow cytometry of intact platelets confirmed trypsin-sensitive SNAP-23 and syntaxin-2 localization to the extracellular surface of the plasma membrane. Acyl-protein thioesterase 1 and botulinum toxin C light chain released SNAP-23 and syntaxin-2, respectively, from the surface of intact platelets. When resting platelets were incubated with both acyl-protein thioesterase 1 and botulinum toxin C light chain, a complex that included both SNAP-23 and syntaxin-2 was detected in supernatants, indicating that extracellular SNARE proteins retain their ability to bind one another. These observations represent the first description of SNARE proteins on the extracellular surface of a cell.

Protease-activated receptors differentially regulate human platelet activation through a phosphatidic acid-dependent pathway

Pathological conditions such as coronary artery disease are clinically controlled via therapeutic regulation of platelet activity. Thrombin, through protease-activated receptor (PAR) 1 and PAR4, plays a central role in regulation of human platelet function in that it is known to be the most potent activator of human platelets. Currently, direct thrombin inhibitors used to block platelet activation result in unwanted side effects of excessive bleeding. An alternative therapeutic strategy would be to inhibit PAR-mediated intracellular platelet signaling pathways. To elucidate the best target, we are studying differences between the two platelet thrombin receptors, PAR1 and PAR4, in mediating thrombin's action. In this study, we show that platelet activation by PAR1-activating peptide (PAR1-AP) requires a phospholipase D (PLD)-mediated phosphatidic acid (PA) signaling pathway. We show that this PAR1-specific PA-mediated effect is not regulated through differential granule secretion after PAR-induced platelet activation. Perturbation of this signaling pathway via inhibition of lipid phosphate phosphatase-1 (LPP-1) by propranolol or inhibition of the phosphatidylcholine-derived phosphatidic acid (PA) formation by PLD with a primary alcohol significantly attenuated platelet activation by PAR1-AP. Platelet activation by thrombin or PAR4-AP was insensitive to these inhibitors. Furthermore, these inhibitors significantly attenuated activation of Rap1 after stimulation by PAR1-AP but not thrombin or PAR4-AP. Because PA metabolites such as diacylglycerol play an important role in intracellular signaling, identifying crucial differences in PA regulation of PAR-induced platelet activation may lead to a greater understanding of the role of PAR1 versus PAR4 in progression of thrombosis.

N-ethylmaleimide-sensitive factor: a redox sensor in exocytosis

Vascular injury triggers endothelial exocytosis of granules, releasing pro-inflammatory and pro-thrombotic mediators into the blood. Nitric oxide (NO) and reactive oxygen species (ROS) limit vascular inflammation and thrombosis by inhibiting endothelial exocytosis. NO decreases exocytosis by regulating the activity of the N-ethylmaleimide-sensitive factor (NSF), a central component of the exocytic machinery. NO nitrosylates specific cysteine residues of NSF, thereby inhibiting NSF disassembly of the soluble NSF attachment protein receptor (SNARE). NO also modulates exocytosis of other cells; for example, NO regulates platelet activation by inhibiting alpha-granule secretion from platelets. Other radicals besides NO can regulate exocytosis as well. For example, H(2)O(2) inhibits exocytosis by oxidizing NSF. Using site-directed mutagenesis, we have defined the critical cysteine residues of NSF, and found that one particular cysteine residue, C264, renders NSF sensitive to oxidative stress. Since radicals such as NO and H(2)O(2) inhibit NSF and decrease exocytosis, NSF may act as a redox sensor, modulating exocytosis in response to changes in oxidative stress.

Essential Role of Protein Kinase Cδ in Platelet Signaling, αIIbβ3 Activation, and Thromboxane A2 Release

The protein kinase C (PKC) family is an essential signaling mediator in platelet activation and aggregation. However, the relative importance of the major platelet PKC isoforms and their downstream effectors in platelet signaling and function remain unclear. Using isolated human platelets, we report that PKCdelta, but not PKCalpha or PKCbeta, is required for collagen-induced phospholipase C-dependent signaling, activation of alpha(IIb)beta(3), and platelet aggregation. Analysis of PKCdelta phosphorylation and translocation to the membrane following activation by both collagen and thrombin indicates that it is positively regulated by alpha(IIb)beta(3) outside-in signaling. Moreover, PKCdelta triggers activation of the mitogen-activated protein kinase-kinase (MEK)/extracellular-signal regulated kinase (ERK) and the p38 MAPK signaling. This leads to the subsequent release of thromboxane A(2), which is essential for collagen-induced but not thrombin-induced platelet activation and aggregation. This study adds new insight to the role of PKCs in platelet function, where PKCdelta signaling, via the MEK/ERK and p38 MAPK pathways, is required for the secretion of thromboxane A(2).

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

The stimulus-induced tyrosine phosphorylation of Munc18c facilitates vesicle exocytosis

Stimulus-induced tyrosine phosphorylation of Munc18c was investigated as a potential regulatory mechanism by which the Munc18c-Syntaxin 4 complex can be dissociated in response to divergent stimuli in multiple cell types. Use of [(32)P]orthophosphate incorporation, pervanadate treatment, and phosphotyrosine-specific antibodies demonstrated that Munc18c underwent tyrosine phosphorylation. Phosphorylation was apparent under basal conditions, but levels were significantly increased within 5 min of glucose stimulation in MIN6 beta cells. Tyrosine phosphorylation of Munc18c was also detected in 3T3L1 adipocytes and increased with insulin stimulation, suggesting that this may be a conserved mechanism. Syntaxin 4 binding to Munc18c decreased as Munc18c phosphorylation levels increased in pervanadate-treated cells, suggesting that phosphorylation dissociates the Munc18c-Syntaxin 4 complex. Munc18c phosphorylation was localized to the N-terminal 255 residues. Mutagenesis of one residue in this region, Y219F, significantly increased the affinity of Munc18c for Syntaxin 4, whereas mutation of three other candidate sites was without effect. Moreover, Munc18c-Y219F expression in MIN6 cells functionally inhibited glucose-stimulated SNARE complex formation and insulin granule exocytosis. These data support a novel and conserved mechanism for the dissociation of Munc18c-Syntaxin 4 complexes in a stimulus-dependent manner to facilitate the increase in Syntaxin 4-VAMP2 association and to promote vesicle/granule fusion.

Regulation of exocytosis by protein kinase C

PKC (protein kinase C) has been known for many years to modulate regulated exocytosis in a wide variety of cell types. In neurons and neuroendocrine cells, PKC regulates several different stages of the exocytotic process, suggesting that these multiple actions of PKC are mediated by phosphorylation of distinct protein targets. In recent years, a variety of exocytotic proteins have been identified as PKC substrates, the best characterized of which are SNAP-25 (25 kDa synaptosome-associated protein) and Munc18. In the present study, we review recent evidence suggesting that site-specific phosphorylation of SNAP-25 and Munc18 by PKC regulates distinct stages of exocytosis.

Complexin II facilitates exocytotic release in mast cells by enhancing Ca2+ sensitivity of the fusion process

Recent studies have shown that soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) proteins are involved in exocytotic release in mast cells as in neurotransmitter release. However, the roles of the proteins that regulate the structure and activity of SNARE proteins are poorly understood. Complexin is one such regulatory protein and is involved in neurotransmitter release, although ideas about its role are still controversial. In this study, we investigated the expression and role of complexin in the regulation of exocytotic release (degranulation) in mast cells. We found that complexin II, but not complexin I, is expressed in mast cells. We obtained RBL-2H3 cells that expressed a low level of complexin II and found that antigen-induced degranulation was suppressed in these cells. No significant changes in the Ca2+ response or expression levels of syntaxins and synaptotagmin were observed in knockdown cells. An immunocytochemical study revealed that complexin II was distributed throughout the cytoplasm before antigen stimulation. However, the distribution of complexin II changed dramatically with stimulation and it became localized on the plasma membrane. This change in the intracellular distribution was observed even in the absence of extracellular Ca2+, while exocytotic release was inhibited almost completely under this condition. The degranulation induced by phorbol 12-myristate 13-acetate and A23187 depended on the extracellular Ca2+ concentration, and its sensitivity to Ca2+ was decreased in knockdown cells. These results suggest that complexin II regulates exocytosis positively by translocating to the plasma membrane and enhancing the Ca2+ sensitivity of fusion machinery, although this translocation to the plasma membrane is not sufficient to trigger exocytotic membrane fusion.

Adipocytes from Munc18c-null mice show increased sensitivity to insulin-stimulated GLUT4 externalization

Insulin-stimulated glucose uptake in adipocytes is mediated by translocation of vesicles containing the glucose transporter GLUT4 from intracellular storage sites to the cell periphery and the subsequent fusion of these vesicles with the plasma membrane, resulting in the externalization of GLUT4. Fusion of the GLUT4-containing vesicles with the plasma membrane is mediated by a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex consisting of vesicle-associated membrane protein 2 (VAMP2), 23-kDa synaptosomal-associated protein (SNAP23), and syntaxin4. We have now generated mouse embryos deficient in the syntaxin4 binding protein Munc18c and show that the insulin-induced appearance of GLUT4 at the cell surface is enhanced in adipocytes derived from these Munc18c-/- mice compared with that in Munc18c+/+ cells. Wortmannin, an inhibitor of PI3K, inhibited insulin-stimulated GLUT4 externalization, without affecting GLUT4 translocation to the cell periphery, in Munc18c+/+ adipocytes, but it did not affect GLUT4 externalization in Munc18c-/- cells. Phosphatidylinositol 3-phosphate, which induced GLUT4 translocation to the cell periphery without externalization in Munc18c+/+ cells, elicited GLUT4 externalization in Munc18c-/- cells. These findings demonstrate that Munc18c inhibits insulin-stimulated externalization of GLUT4 in a wortmannin-sensitive manner, and they suggest that disruption of the interaction between syntaxin4 and Munc18c in adipocytes might result in enhancement of insulin-stimulated GLUT4 externalization.

Presence of SNAP-25 in rat mast cells

Mast cells participate in inflammation and allergies by releasing active mediators stored in numerous cytoplasmic granules. Degranulation implies compound exocytosis which involves a combination of granule-granule and granule-plasma membrane fusions. One of the most important proteins in the exocytotic process in neural and endocrine cells is the synaptosomal associated protein of 25 kDa (SNAP-25). In the present study, using a highly specific monoclonal antibody against SNAP-25, we have demonstrated by immunocytochemistry, western blot and immunoelectron microscopy the presence of SNAP-25 in rat peritoneal mast cells. Likewise we localized the protein mainly on the membrane of the secretory granules. Thus while the precise function of SNAP-25 in mast cells remains to be elucidated, it may be envolved in granule-granule fusion needed in degranulation.

A role for SNAP-25 but not VAMPs in store-mediated Ca2+ entry in human platelets

Store-mediated Ca2+ entry (SMCE) is a major mechanism for Ca2+ influx in non-excitable cells. Recently, a conformational coupling mechanism allowing coupling between transient receptor potential channels (TRPCs) and IP3 receptors has been proposed to activate SMCE. Here we have investigated the role of two soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs), which are involved in membrane trafficking and docking, in SMCE in human platelets. We found that the synaptosome-associated protein (SNAP-25) and the vesicle-associated membrane proteins (VAMP) coimmunoprecipitate with hTRPC1 in platelets. Treatment with botulinum toxin (BoNT) E or with tetanus toxin (TeTx), induced cleavage and inactivation of SNAP-25 and VAMPs, respectively. BoNTs significantly reduced thapsigargin- (TG) and agonist-evoked SMCE. Treatment with BoNTs once SMCE had been activated decreased Ca2+ entry, indicating that SNAP-25 is required for the activation and maintenance of SMCE. In contrast, treatment with TeTx had no effect on either the activation or the maintenance of SMCE in platelets. Finally, treatment with BoNT E impaired the coupling between naturally expressed hTRPC1 and IP3 receptor type II in platelets. From these findings we suggest SNAP-25 has a role in SMCE in human platelets.

Phosphorylation of SNAP-23 in activated human platelets

Phosphorylation of SNARE proteins may provide a critical link between cell activation and secretory processes. Platelets contain all three members of the SNAP-23/25/29 gene family, but by comparison to brain tissue, SNAP-23 is the most highly enriched of these proteins in platelets. SNAP-23 function is required for exocytosis from platelet alpha, dense, and lysosomal granules. SNAP-23 was phosphorylated largely on serine residues in platelets activated with thrombin. Phosphorylation kinetics paralleled or preceded granule secretion. Inhibition studies suggested that SNAP-23 phosphorylation proceeds largely through a protein kinase C (PKC) mechanism and purified PKC directly phosphorylated recombinant (r-) SNAP-23 (up to 0.3 mol of phosphate/mol of protein). Five major tryptic phosphopeptides were identified in cellular SNAP-23 isolated from activated platelets; three phosphopeptides co-migrated with those identified in PKC-phosphorylated r-SNAP-23. In contrast, only one major phosphopeptide was identified when SNAP-23, engaged in a ternary SNARE complex, was phosphorylated by PKC. Ion trap mass spectrometry revealed that platelet SNAP-23 was phosphorylated at Ser23/Thr24 and Ser161, after cell activation by thrombin; these sites were also identified in PKC-phosphorylated r-SNAP-23. SNAP-23 mutants that mimic phosphorylation at Ser23/Thr24 inhibited syntaxin 4 interactions, whereas a phosphorylation mutant of Ser161 had only minor effects. Taken together these studies show that SNAP-23 is phosphorylated in platelets during cell activation through a PKC-related mechanism at two or more sites with kinetics that parallel or precede granule secretion. Because mutants that mimic SNAP-23 phosphorylation affect syntaxin 4 interactions, we hypothesize that SNAP-23 phosphorylation may be important for modulating SNARE-complex interactions during membrane trafficking and fusion.

Molecular basis of platelet granule secretion

The energy-dependent release of granule contents from activated platelets is a well-established component of normal hemostasis and thrombosis. A role for membrane fusion in this process has been presumed for decades, but only recently have the mechanisms of platelet membrane fusion been investigated at the molecular level. Such studies have demonstrated that platelet membrane fusion is controlled by lipid components of the membrane bilayer, by transmembrane proteins termed SNARE proteins, and by chaperone proteins that interact with SNARE proteins. This core membrane fusion machinery is controlled by activation-dependent changes in cytoskeletal organization, intracellular calcium levels, kinase activity, and intracellular protease activity. Through these mechanisms, interactions of ligands with their cognate cell-surface receptors are transmitted to the membrane fusion machinery to facilitate membrane fusion and secretion of granule contents from platelets.

Munc18-syntaxin complexes and exocytosis in human platelets

The Sec1-Munc18 (SM) proteins are required for cellular exocytosis, but their mechanistic function remains poorly understood. We examined SM-syntaxin complexes in human platelets, which are terminally differentiated, anuclear cells that secrete the contents of their intracellular granules through syntaxin 2- and syntaxin 4-dependent mechanisms. Munc18a, Munc18b, and Munc18c were detected in human platelets by immunoblotting and/or PCR. The SM proteins and syntaxin 2 were found in the membrane and cytosolic fractions of cells, whereas syntaxin 4 was detected only in the membrane. Platelet membranes contain Munc18c-syntaxin 4 complexes, but minimal if any Munc18c-syntaxin 2 complexes were found. No significant amounts of Munc18a or Munc18b complexes were seen with either syntaxin. Munc18c-syntaxin 4 complexes were dissociated when cells were activated to secrete. Two potential inhibitors of Munc18c-syntaxin 4 complexes were generated to examine whether complex dissociation may lead to exocytosis. Peptides that mimic the projected intermolecular contact sites of Munc18c with syntaxin enhanced Ca2+-triggered dense granule exocytosis in permeabilized cells. Similarly, an anti-Munc18c monoclonal antibody that inhibited the Munc18c-syntaxin complex potently amplified Ca2+-induced platelet granule secretion. In summary, Munc18 proteins bind to specific syntaxin isoforms in platelets despite the presence of other potential binding partners. Acute inhibition of the SM-syntaxin complex promotes Ca2+-induced exocytosis, suggesting that complex formation per se has a regulatory effect on triggered secretion.