Platelet Phosphatidylserine Research Articles

Distinct Subtypes of Microparticle-containing Immune Complexes Are Associated with Disease Activity, Damage, and Carotid Intima-media Thickness in Systemic Lupus Erythematosus.

Microparticles (MP) are small extracellular vesicles present in body fluids. MP originate from different cellular lineages, principally from platelets in blood, and may expose phosphatidylserine (PS). In systemic lupus erythematosus (SLE), MP harbor immunoglobulin G (IgG), thereby forming MP-containing immune complexes (mpIC). We aimed to verify an association between SLE disease activity, damage, and surrogate markers of atherosclerosis and MP harboring IgG, taking into account the platelet origin and PS exposure of MP. MP expressing surface IgG, platelet antigen (CD41+), and PS were quantified using flow cytometry in plasma of 191 women with SLE. Carotid ultrasounds (US) were available in 113 patients. Spearman correlation analysis was used to analyze whether levels of MP were associated with the following outcomes: SLE Disease Activity Index 2000 (SLEDAI-2K), Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index (SDI), and carotid US plaques and intima-media thickness (CIMT) as surrogates for vascular damage. We found CD41+ MP harboring IgG present in SLE. A positive correlation was found between SLEDAI-2K and levels of CD41+ MP harboring IgG and exposing (p = 0.027) and non-exposing PS (p = 0.001). Conversely, SDI (p = 0.024) and CIMT (p = 0.016) correlated with concentrations of CD41- MP harboring IgG and exposing PS. Associations were independent of low-density lipoprotein cholesterol level, body mass index, and antimalarial drug use. Different subtypes of mpIC are produced in SLE and are associated with distinct clinical characteristics such as disease activity and vascular damage. The assessment of MP subtypes might serve for the design of predictive markers of disease activity and vascular damage in patients.

Protease‐activated receptor 4 is more important than protease‐activated receptor 1 for the thrombin‐induced procoagulant effect on platelets

Protease-activated receptor 4 is more important than protease-activated receptor 1 for the thrombin-induced procoagulant effect on platelets

Abstract 42: Genetic Ablation of TMEM16F Exhibits Strain-specific Lethality in Mice

Background: Scott Syndrome is a rare bleeding disorder characterized by a defect in platelet phosphatidylserine (PS) exposure. The syndrome has recently been linked to mutations in TMEM16F. Tmem16f -/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding. We now report analysis of an additional gene targeted Tmem16f allele generated in C57BL/6 ES cells. Results: JM8 ES cell were obtained from EUCOMM, and successful Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f +/gt (+/gt) intercross progeny identified no surviving Tmem16f gt/gt (gt/gt) mice at weaning (p<0.001). However, +/gt intercrosses generated the expected Mendelian genotype ratios at both E10.5 and E17.5, with gt/gt embryo’s exhibiting no morphological abnormalities on gross or routine histologic examination. Though complete deficiency of TMEM16F is lethal in the C57BL/6J genetic background between E17.5 and birth, an F2 intercross of +/gt mice outcrossed one generation to 129x1SvJ resulted in gt/gt mice surviving to weaning, though at reduced numbers (6/75 total progeny compared to ~19 expected, p <0.002). Progeny testing of surviving gt/gt mice suggest a single autosomal dominant 129x1SvJ-associated genetic modifier. Preliminary genetic analysis of these mice appears to map this locus to the proximal region of chromosome 3. Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p<0.05). Notably, platelets from +/gt mice exhibited a trend toward reduced PS exposure, detected with FITC-labelled lactadherin, in response to PAR4 agonist peptide, whereas gt/gt mice had significantly reduced PS exposure (p < 0.05). Conclusion: These data suggest the existence of a viability-determining genetic modifier of TMEM16F in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover novel functions for TMEM16F and the regulation of hemostatic function.

ATP‐binding cassette transporter 1 (ABCA1) deficiency decreases platelet reactivity and reduces thromboxane A2 production independently of hematopoietic ABCA1

ESSENTIALS: The role of ATP-binding cassette transporter 1 (ABCA1) in platelet functions is poorly characterized. We studied the impact of ABCA1 deficiency on platelet responses in a mouse model and two Tangier patients. ABCA1-deficient platelets exhibit reduced positive feedback loop mechanisms. This reduced reactivity is dependent on external environment and independent of hematopoietic ABCA1. The ATP-binding cassette transporter ABCA1 is required for the conversion of apolipoprotein A-1 to high-density lipoprotein (HDL), and its defect causes Tangier disease, a rare disorder characterized by an absence of HDL and accumulation of cholesterol in peripheral tissues. The role of ABCA1 in platelet functions remains poorly characterized. To determine the role of ABCA1 in platelet functions and to clarify controversies concerning its implication in processes as fundamental as platelet phosphatidylserine exposure and control of platelet membrane lipid composition. We studied the impact of ABCA1 deficiency on platelet responses in a mouse model and in two Tangier patients. We show that platelets in ABCA1-deficient mice are slightly larger in size and exhibit aggregation and secretion defects in response to low concentrations of thrombin and collagen. These platelets have normal cholesterol and major phospholipid composition, granule morphology, or calcium-induced phosphatidylserine exposure. Interestingly, ABCA1-deficient platelets display a reduction in positive feedback loop mechanisms, particularly in thromboxane A2 (TXA2) production. Hematopoietic chimera mice demonstrated that defective eicosanoids production, particularly TXA2, was primarily dependent on external environment and not on the hematopoietic ABCA1. Decreased aggregation and production of TXA2 and eicosanoids were also observed in platelets from Tangier patients. Absence of ABCA1 and low HDL level induce reduction of platelet reactivity by decreasing positive feedback loops, particularly TXA2 production through a hematopoietic ABCA1-independent mechanism.

MARCKS ED Inhibits Fibrin Formation By Blocking Coagulation Protein Complex Assembly on Phosphatidylserine

Introduction: Prothrombinase is an essential enzyme of the coagulation cascade that is made up of a heterodimer of coagulation factor Xa and Va. The complex most readily forms in the presence of lipid membranes containing the anionic lipid phosphatidylserine. The phosphatidylserine lipid head groups provide a surface for the prothrombinase complex to assemble and significantly increase the enzyme's turnover number. Here, we investigate how truncation and modification of the naturally occurring myristoylated alanine-rich C-kinase substrate (MARCKS) protein into small peptide and peptidomimetic fragments (MARCKS ED) forms lipid probes that bind to phosphatidylserine, and block the assembly and enzymatic activity of the prothrombinase complex. Compared to known phosphatidylserine binding proteins such as annexin V or lactadherin, the MARCKS ED probes are less than a tenth of the molecular weight and do not require calcium to bind to phosphatidylserine.Methods: Using biophysical assays, we examine how the MARCKS ED probes assemble on lipid membranes containing phosphatidylserine and how this assembly alters the binding of factor Xa to these surfaces. Effects on the enzymatic activity of the prothrombinase complex in the presence of activated platelets and other phosphatidylserine exposing membranes are explored using a modified prothrombinase assay. The binding and anticoagulant activity of the MARCKS ED probes during whole blood coagulation are examined by quantifying fluorescently labeled MARCKS ED probe localization, platelet accumulation, and fibrin formation in microfluidic flow assays.Results: We find that the MARCKS ED probes can antagonize factor Xa from binding phosphatidylserine by measuring reduced association of factor Xa to lipid membranes treated with the MARCKS ED probes. Pre-treatment of activated platelets with the MARCKS ED probes significantly reduced the assembly and enzymatic activity of prothrombinase (100% activity with vehicle control vs. 51% with 1 μM MARCKS ED peptide treatment, P < 0.05). Similar inhibitory activity was also observed when activated platelets were replaced with other phosphatidylserine exposing lipid membranes, such as synthetic liposomes or biologic microparticles. In the whole blood microfluidic flow assay, the MARCKS ED probes colocalized with adherent platelets and inhibited fibrin formation (100% peak fibrin intensity with vehicle control vs. 17% peak fibrin intensity with 1 μM MARCKS ED peptide treatment, P < 0.05). The MARCKS ED probes did not alter platelet surface area coverage. Electron microscopy images of the final clots formed in the microfluidic flow assay show an absence of fibrin formation with MARCKS ED probe treatment.Conclusions: We demonstrate that the MARCKS ED probes can inhibit the assembly and enzymatic activity of prothrombinase, and that this activity translates to significantly reduced fibrin formation in whole blood. The MARCKS ED peptide and peptidomimetic probes may provide a new therapeutic tool for inhibiting the assembly of phosphatidylserine dependent coagulation factors, as well as a research tool for identifying platelet phosphatidylserine exposure. DisclosuresKastelowitz:University of Colorado Boulder: Patents & Royalties. Yin:University of Colorado Boulder: Patents & Royalties.

Platelet Phosphatidylserine Exposure, Survival and Blood Coagulation in Mice Lacking TMEM16F

Scott Syndrome is a rare, moderately severe bleeding disorder caused by a defect in platelet, red cell, and lymphocyte phosphatidylserine exposure. The syndrome has been linked to mutations in TMEM16F, a Ca++-activated ion channel. A moderately severe bleeding disorder in German Shephard dogs, characterized by decreased platelet phosphatidylserine exposure, has also linked to mutations in TMEM16F.TMEM16F, is a member of a recently-identified family of calcium-activated chloride channels that are also called anoctamins. Members of the family apparently serve both as ion channels and phospholipid scrambling channels. A crystal structure of one member of the TMEM16 family shows a homo-dimeric structure of an integral membrane protein with each unit containing 10 transmembrane helices. A transmembrane hydrophilic cavity lies at the interface, and contains a cation binding site, and a slot with dimensions that could accommodate acyl chains.Tmem16f-/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding. We have developed an independent gene-targeted (gt) Tmem16f allele generated in C57BL/6 ES cells and find results that both confirm prior reports and contrast with them.JM8 ES cells were obtained from EUCOMM, and Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f+/gt intercross progeny identified no surviving Tmem16fgt/gt mice at weaning (p<0.001). However, +/gt intercrosses generated the expected Mendelian genotype ratios at both E10.5 and E17.5, with gt/gt embryos. Blinded pathological evaluation of E17.5 gt/gt pups indicated reduction of ossification and angular limb changes, consistent with 2 prior reports. Thus, our results confirm previously reported bone changes, but in contrast with prior reports, indicate that Tmem16f deficiency is lethal in the C57BL/6 genetic background.An F2 intercross of +/gt mice outcrossed one generation to 129x16vJ resulted in gt/gt mice surviving to weaning, though at only 30% of the expected Mendelian frequency. Progeny analysis points to a single autosomal dominant 129x1SvJ-associated genetic modifier. Preliminary genetic analysis of these mice appears to map this locus to the proximal region of chromosome 3. Further efforts to localize the responsible gene(s) are underway.Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p<0.05). Notably, platelets from +/gt mice exhibited a trend toward reduced PS exposure, detected with FITC-labelled lactadherin, in response to PAR4 agonist peptide, whereas gt/gt mice had significantly reduced PS exposure (p < 0.05). gt/gt platelets showed a trend toward reduced PS exposure in response to A23187, as well as prolonged platelet rich plasma clotting times, and less efficient lactadherin inhibition of platelet clotting time.Our data suggest the existence of a viability-determining genetic modifier of the TMEM16F deficiency phenotype in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover a novel regulator of hemostatic function. The observation that heterozygous deficiency leads to a PS exposure and hemostatic phenotype also suggests the possibility that heterozygous TMEM16F mutations may influence hemostasis or thrombosis in humans. DisclosuresNo relevant conflicts of interest to declare.

A TMEM16F point mutation causes an absence of canine platelet TMEM16F and ineffective activation and death‐induced phospholipid scrambling

TMEM16F is an ion channel and calcium-dependent lipid scramblase that mediates phosphatidylserine (PS) exposure on the plasma membrane. Two disparate disease phenotypes are associated with TMEM16F loss-of-function mutations: a rare bleeding disorder (Scott syndrome) and skeletal malformations due to aberrant bone mineralization in a TMEM16F knockout mouse. We therefore undertook comparative studies of TMEM16F expression in canine Scott syndrome (CSS), an autosomal recessive platelet defect. To define anoctamin proteins and scramblase response of CSS platelets and to determine whether TMEM16F is the CSS disease gene. CSS TMEM16F cDNA and gene were sequenced and mutation detection was performed in CSS pedigrees. Platelet fractions from CSS dogs were isolated for proteomic and immunologic characterization of TMEM16F. Annexin V was used as a flow cytometric marker of induced platelet PS externalization. A TMEM16F splice site mutation segregated with the CSS trait and TMEM16F protein was undetectable in CSS platelet membranes; however, a second anoctamin, TMEM16K, was found. Proteomic analyses revealed a network of 32 proteins that differentially cosegregated with platelet plasma membrane TMEM16F. CSS platelets had profoundly impaired scramblase response to pharmacologic and physiologic agents that increase intraplatelet calcium and conditions that induce apoptotic and necrotic cell death. CSS platelets represent a TMEM16F-null mutant model that demonstrates a central role for TMEM16F in mediating platelet PS externalization in response to activating and death signals. Platelet TMEM16F may prove a novel drug target for modulating platelet procoagulant activity and extending platelet life span.

Desmopressin treatment improves platelet function under flow in patients with postoperative bleeding

Patients undergoing major cardiothoracic surgery are subjected to dilution, owing to massive fluid infusion and blood component transfusion. These patients may experience bleeding perioperatively, and are frequently treated with the endothelium-activating agent desmopressin. To investigate the effect of desmopressin administration on von Willebrand factor (VWF)-dependent coagulant and platelet functions under flow conditions. Blood from 16 patients with postoperative bleeding was obtained before and after desmopressin treatment (0.3μgkg(-1) body weight), and assessed for coagulant properties and platelet function. Furthermore, VWF antigen levels and multimer composition were determined in both samples. Desmopressin treatment did not change thrombin generation in plasma or whole blood thromboelasticity. Also coagulation factor levels (other than factorVIII) and coagulation times were unchanged, suggesting that desmopressin treatment did not have a major effect on the coagulant activity. On the other hand, desmopressin treatment raised the already high plasma levels of VWF from a median of 116IUmL(-1) (interquartile range [IQR]102-154IUmL(-1) ) to a median of 160IUmL(-1) (IQR126-187IUmL(-1) ) (P=0.007), owing to accumulation of the high molecular weight VWF multimers. Furthermore, desmopressin treatment caused an increase in collagen-dependent thrombus formation and platelet phosphatidylserine exposure. Markers of thrombus formation correlated with the plasma levels of VWF. Invitro control experiments confirmed a major contribution of VWF to thrombus formation and procoagulant activity under conditions of blood dilution. Desmopressin treatment of patients with bleeding complications after cardiothoracic surgery induces the release of high molecular weight VWF multimers, which enhance platelet activation and thrombus formation under flow conditions.

Rate-limiting roles of the tenase complex of factors VIII and IX in platelet procoagulant activity and formation of platelet-fibrin thrombi under flow.

The importance of factor Xa generation in thrombus formation has not been studied extensively so far. Here, we used mice deficient in either factor VIII or factor IX to determine the role of platelet-stimulated tenase activity in the formation of platelet-fibrin thrombi on collagen. With tissue factor present, deficiency in factor VIII or IX markedly suppressed thrombus growth, fibrin formation and platelet procoagulant activity (phosphatidylserine exposure). In either case, residual fibrin formation was eliminated in the absence of tissue factor. Effects of factor deficiencies were antagonized by supplementation of the missing coagulation factor. In wild-type thrombi generated under flow, phosphatidylserine-exposing platelets bound (activated) factor IX and factor X, whereas factor VIII preferentially co-localized at sites of von Willebrand factor binding. Furthermore, proteolytic activity of the generated activated factor X and thrombin was confined to the sites of phosphatidylserine exposure. With blood from a hemophilia A or B patient, the formation of platelet-fibrin thrombi was greatly delayed and reduced, even in the presence of high concentrations of tissue factor. A direct activated factor X inhibitor, rivaroxaban, added to human blood, suppressed both thrombin and fibrin formation. Together, these data point to a potent enforcement loop in thrombus formation due to factor X activation, subsequent thrombin and fibrin generation, causing activated factor X-mediated stimulation of platelet phosphatidylserine exposure. This implies that the factor VIII/factor IX-dependent stimulation of platelet procoagulant activity is a limiting factor for fibrin formation under flow conditions, even at high tissue factor concentrations.

Platelets: the unsung hero of the immune response

Platelets: the unsung hero of the immune response

Platelets interact with Coxsackieviruses B and have a critical role in the pathogenesis of virus‐induced myocarditis

To further understand the role of platelets in the pathogenesis of viral infections we explored platelet interaction with Coxsackieviruses B (CVB) 1 and 3. CVB is a group of viruses that cause the majority of human enterovirus-related viral myocarditis; their receptor (CAR) is expressed on the platelet surface and there is a well-characterized CVB3-induced myocarditis murine model. Human platelets were infected with CVB1 and 3 and viruses were detected in pellets and in supernatants. C57BL/6J mice with or without platelet depletion were inoculated with CVB3 and peripheral blood and heart samples collected at different times post-infection. CVB1 and 3 RNA and a capsid protein were detected in infected platelets. Despite the fact that titration assays in Vero cells showed increasing infectivity titers over time, supernatants and pellets from infected platelets showed similar levels, suggesting that platelets were not susceptible to a replicative infectivity cycle. CVB binding was CAR-independent and resulted in P-selectin and phosphatidylserine (PS) exposure. CVB3-infected mice showed a rapid thrombocytopenia that correlated with an increase in platelet PS exposure and platelet-leukocyte aggregates without modification of platelet P-selectin expression or von Willebrand factor levels. Mortality, viremia, heart viral titers and myocarditis were significantly higher in platelet-depleted than normal animals. Type I IFN levels were not changed but IgG levels were lower in infected and platelet-depleted mice. Our data reveal that platelets play a critical role in host survival and immune response against CVB3 infection.

X-Ray Structure of a Calcium Activated TMEM16 Lipid Scramblase

The TMEM16s or anoctamins constitute a class of eukaryotic membrane proteins that in mammals contain ten members with high sequence conservation. Despite their close relationship these proteins are characterized by a remarkable functional diversity. The family includes the long sought-after Ca2+-activated chloride channels (TMEM16A and B) but also cation channels and lipid scramblases that support the exchange of lipids between the inner and outer leaflets of the bilayer in an ATP-independent manner. As part of the blood coagulation process, TMEM16F triggers the exposure of phosphatidylserine in blood platelets upon activation by Ca2+. TMEM16C, D, G and J were suggested to work as scramblases as well, but with variable characteristics. Although we have by now gained considerable insight into the functional properties of certain family members, their architecture and the relation to mechanisms of action were so far unknown. Here we present the crystal structure of nhTMEM16, a fungal family member that operates as a Ca2+-activated lipid scramblase. Each subunit of the homodimeric protein contains ten transmembrane helices and a hydrophilic membrane-traversing cavity that resembles a ‘spiral staircase’ and is exposed to the lipid bilayer as a potential site of catalysis. This cavity harbors a conserved Ca2+-binding site located within the hydrophobic core of the membrane. Ca2+ binding by six residues, five of which carry a negative charge, controls the activation of scrambling in nhTMEM16. A triple mutant of residues in this site shows only weak scrambling activity that is no longer enhanced by Ca2+.The structure thus reveals the general architecture of the family and its mode of Ca2+-activation. It provides insights into potential scrambling mechanisms and will furthermore serve as a framework to unravel the conduction of ions in certain TMEM16 proteins.

Platelet Mechanosensing of Collagen Matrices

Background: Vascular injury causes platelets to initiate hemostasis by first adhering to exposed subendothelial matrix proteins such as collagen. While the biochemical and biological aspects of platelet adhesion via collagen and von willebrand factor are well characterized, if and whether the mechanical properties of the subendothelial matrix affect platelet function is relatively unknown. As purely mechanical cues, such as substrate stiffness, from collagen matrices are sensed and transduced by endothelial cells to alter their physiological processes, platelets may also exhibit similar behavior (Chein S., AM J Physiol Heart Circ Physiol, 2007). In addition, recent reports demonstrate that the stiffness of the subendothelium varies in different disease states, including atherosclerosis and changes associated with aging (Stroka KM & Aranda-Espinoza H, Blood, 2011). Therefore, understanding the effect of subendothelium stiffness on platelet adhesion, spreading, and activation will provide insight into how biomechanical and biochemical factors interact during clot formation in health and disease. To that end, we fabricated polyacrylamide (PA) gels of varying stiffnesses covalently conjugated with collagen to model the biomechanical properties of the subendothelium. Via differential crosslinking, the stiffness of these PA gels can be varied while maintaining constant collagen concentrations on the gel surface, enabling decoupling of mechanical and biochemical cues (Lam, et al, Mol Cancer, 2010; Pathak A & Kumar S, P Natl Acad Sci, 2012). With these systems, we investigated how the stiffness of the underlying collagen matrix affects platelet adhesion, spreading and activation under static and flow conditions and used pharmacological cytoskeletal inhibitors to investigate the underlying mechanotransductive mechanisms.Results and Discussions: Type I collagen was covalently conjugated to the surface of PA gels with varying substrate stiffnesses (0.25 to 100 kilopascals (kPa)); the range of vessel wall stiffness in vivo. The PA gels were then incubated with 5.5 x 106 platelets/ml for 1 hour and stained with a fluorescent membrane dye. While collagen substrate stiffness did not affect platelet adhesion, determined by the number of platelets on each gel, significant differences were observed in platelet spreading area on collagen-conjugated PA gels with stiffnesses >2.5 kPa compared to PA gels of <2.5 kPa (Fig. 1A and 1B). Using PAC-1-FITC and Annexin-V-AF488 to measure platelet integrin αIIbβ3 activation and platelet phosphatidylserine (PS) exposure, respectively, we found that varying the substrate stiffness of collagen matrices did not affect αIIbβ3 activation on adherent platelets but did result in differential levels of PS exposure on adherent platelets, with increased PS exposure on stiffer PA gels (Fig. 1C). In addition, using the myosin light chain kinase (MLCK) inhibitor ML-7 and the Rho kinase inhibitor Y-27632, we observed that platelet exposure to ML-7 eliminated the substrate stiffness-mediated effect on platelet spreading as platelet spreading on PA gels stiffer than 5 kPa was decreased to the levels of that on soft PA gels of 0.5 kPa. Y-27632 exposure did not cause a similar effect, as platelet spreading was increased for all stiffness conditions (Fig. 1D). Finally, under flow conditions using a shear rate of 150 s-1, platelet adhesion in addition to platelet spreading was mediated by substrate stiffness (Fig. 1E); possibly due to weaker collagen attachment on softer substrates that cannot sufficiently resist drag forces.Conclusion: Our results indicate that platelets adhered on collagen mechanosense the stiffness of the underlying subendothelial substrate and transduce these cues into differential levels of adhesion, spreading and activation. While MLCK mediates aspects of this process, further mechanistic studies are currently being conducted. In addition, as shear stress might interact with the observed substrate stiffness-mediated phenomenon, additional experiments under flow at different shear rates are also ongoing. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

TMEM16F Deficiency Causes a Strain-Dependent Lethality in Mice and Reduced Phosphatidylserine Exposure on Heterozygous As Well As Homozygous Deficient Mouse Platelets

Scott Syndrome is a rare bleeding disorder characterized by a defect in platelet phosphatidylserine (PS) exposure. The syndrome has recently been linked to mutations in TMEM16F, a Ca++-activated ion channel. Tmem16f-/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding (Yang et al, Cell 151: 111-122; 2012). We now report analysis of an additional gene targeted Tmem16f allele generated in C57BL/6 ES cells.JM8 ES cell were obtained from EUCOMM, and successful Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f+/gt (+/gt) intercross progeny identified no surviving Tmem16fgt/gt (gt/gt) mice at weaning (p<0.001). However, +/gt intercrosses generated the expected Mendelian genotype ratios at both E10.5 and E17.5, with gt/gt embryo’s exhibiting no morphological abnormalities on gross or routine histologic examination. Though complete deficiency of TMEM16F is lethal in the C57BL/6J genetic background between E17.5 and birth, an F2 intercross of +/gt mice outcrossed one generation to 129x1SvJ resulted in gt/gt mice surviving to weaning, though at reduced numbers (6/75 total progeny compared to ~19 expected, p <0.002). Progeny testing of surviving gt/gt mice suggest a single autosomal dominant 129x1SvJ-associated genetic modifier. Preliminary genetic analysis of these mice appears to map this locus to the proximal region of chromosome 3.Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p<0.05). Notably, platelets from +/gt mice exhibited a trend toward reduced PS exposure, detected with FITC-labelled lactadherin, in response to PAR4 agonist peptide, whereas gt/gt mice had significantly reduced PS exposure (p < 0.05). gt/gt platelets showed a trend toward reduced PS exposure in response to A23187, as well as prolonged platelet rich plasma clotting times, and less efficient lactadherin inhibition of platelet clotting time.These data suggest the existence of a viability-determining genetic modifier of TMEM16F in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover novel functions for TMEM16F and the regulation of hemostatic function. The observation of a PS exposure and hemostatic phenotype in +/gt mice also suggests the possibility that heterozygous TMEM16F mutations may subtly influence hemostasis or thrombosis in humans. DisclosuresNo relevant conflicts of interest to declare.

Agonist-induced platelet procoagulant activity requires shear and a Rac1-dependent signaling mechanism

AbstractActivated platelets facilitate blood coagulation by exposing phosphatidylserine (PS) and releasing microvesicles (MVs). However, the potent physiological agonists thrombin and collagen poorly induce PS exposure when a single agonist is used. To obtain a greater procoagulant response, thrombin is commonly used in combination with glycoprotein VI agonists. However, even under these conditions, only a percentage of platelets express procoagulant activity. To date, it remains unclear why platelets poorly expose PS even when stimulated with multiple agonists and what the signaling pathways are of soluble agonist-induced platelet procoagulant activity. Here we show that physiological levels of shear present in blood significantly enhance agonist-induced platelet PS exposure and MV release, enabling low doses of a single agonist to induce full-scale platelet procoagulant activity. PS exposed on the platelet surface was immediately released as MVs, revealing a tight coupling between the 2 processes under shear. Using platelet-specific Rac1−/− mice, we discovered that Rac1 plays a common role in mediating the low-dose agonist-induced procoagulant response independent of platelet aggregation, secretion, and the apoptosis pathway. Platelet-specific Rac1 function was not only important for coagulation in vitro but also for fibrin accumulation in vivo following laser-induced arteriolar injury.

Abstract 197: p300-Based Acetylation Signaling Regulates Cytoskeletal Reorganization and Procoagulant Phenotype of Platelets

Introduction: Reversible acetylation signaling catalyzed by acetyltransferases and deacetylases regulates multiple cellular events. We recently showed a role for deacetylation signaling in platelet function via the actions of the deacetylase enzyme HDAC6. Here we investigate the role of acetylation mediated by the p300 class of acetyltransferase proteins in platelet function. Methods: To test the hypothesis that p300 acetylation signaling controls cytoskeletal remodeling and the exposure of phosphatidylserine (PS) in platelets, we used cell biological and hematological methods, including enzymatic assays, microscopy, cell sorting, aggregometry and in vitro assays of thrombus formation. Results: Platelets express high levels of p300 protein. Upon stimulation with the glycoprotein VI (GPVI) agonist collagen-related peptide (CRP), total platelet acetyltransferase enzyme activity increases. Pre-treatment of platelets with pharmacological inhibitors of p300 blocks agonist-induced acetyltransferase activity and prevents the acetylation of p300 substrate proteins. Inhibition of p300 blocked the deacetylation of HDAC6 and tubulin and disrupted the platelet tubulin cytoskeleton. p300 inhibition also increased basal platelet PS exposure via a caspase-dependent apoptotic pathway. On a functional level, p300 inhibition prevented platelet shape change, aggregation and spreading while increasing the procoagulant activity of platelets in whole blood. Conclusions: These results suggest that p300-based acetylation signaling regulates separable aspects of platelet function. p300 plays a role in the regulation of the platelet cytoskeleton as well as PS exposure to effect platelet spreading, aggregation, thrombus formation as well as platelet procoagulant activities. Future studies of the platelet acetylome will aim to identify p300 target substrates and specify roles for p300 in in the context of platelet activation, thrombus formation and pathologies of the cardiovascular system. Acknowledgements: This work is funded by the American Heart Association (13POST13730003 to J.E.A. and 13EIA12630000 to O.J.T.M.) and the National Institutes of Health (R01HL101972 to O.J.T.M.).

Chloride channels are necessary for full platelet phosphatidylserine exposure and procoagulant activity

Platelets enhance thrombin generation at sites of vascular injury by exposing phosphatidylserine during necrosis-like cell death. Anoctamin 6 (Ano6) is required for Ca2+-dependent phosphatidylserine exposure and is defective in patients with Scott syndrome, a rare bleeding disorder. Ano6 may also form Cl− channels, though the role of Cl− fluxes in platelet procoagulant activity has not been explored. We found that Cl− channel blockers or removal of extracellular Cl− inhibited agonist-induced phosphatidylserine exposure. However, this was not due to direct inhibition of Ca2+-dependent scrambling since Ca2+ ionophore-induced phosphatidylserine exposure was normal. This implies that the role of Ano6 in Ca2+−dependent PS exposure is likely to differ from any putative function of Ano6 as a Cl− channel. Instead, Cl− channel blockade inhibited agonist-induced Ca2+ entry. Importantly, Cl− channel blockers also prevented agonist-induced membrane hyperpolarization, resulting in depolarization. We propose that Cl− entry through Cl− channels is required for this hyperpolarization, maintaining the driving force for Ca2+ entry and triggering full phosphatidylserine exposure. This demonstrates a novel role for Cl− channels in controlling platelet death and procoagulant activity.

Dynamic adhesion of eryptotic erythrocytes to immobilized platelets via platelet phosphatidylserine receptors

Glucose depletion of erythrocytes triggers suicidal erythrocyte death or eryptosis, which leads to cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes adhere to endothelial cells by a mechanism involving phosphatidylserine at the erythrocyte surface and CXCL16 as well as CD36 at the endothelial cell membrane. Nothing has hitherto been known about an interaction between eryptotic erythrocytes and platelets, the decisive cells in primary hemostasis and major players in thrombotic vascular occlusion. The present study thus explored whether and how glucose-depleted erythrocytes adhere to platelets. To this end, adhesion of phosphatidylserine-exposing erythrocytes to platelets under flow conditions was examined in a flow chamber model at arterial shear rates. Platelets were immobilized on collagen and further stimulated with adenosine diphosphate (ADP, 10 μM) or thrombin (0.1 U/ml). As a result, a 48-h glucose depletion triggered phosphatidylserine translocation to the erythrocyte surface and augmented the adhesion of erythrocytes to immobilized platelets, an effect significantly increased upon platelet stimulation. Adherence of erythrocytes to platelets was blunted by coating of erythrocytic phosphatidylserine with annexin V or by neutralization of platelet phosphatidylserine receptors CXCL16 and CD36 with respective antibodies. In conclusion, glucose-depleted erythrocytes adhere to platelets. The adhesive properties of platelets are augmented by platelet activation. Erythrocyte adhesion to immobilized platelets requires phosphatidylserine at the erythrocyte surface and CXCL16 as well as CD36 expression on platelets. Thus platelet-mediated erythrocyte adhesion may foster thromboocclusive complications in diseases with stimulated phosphatidylserine exposure of erythrocytes.

TMEM16F Plays a Vital Role In Platelet Microparticle Generation and Thrombosis

Transmembrane (TMEM)16f is member of the TMEM16 family of ion channels, recently shown to be essential for optimal Ca2+-dependent phospholipid scrambling in platelets, platelet-dependent pro-coagulant activity and thrombosis. However, relatively little is known about the effect of TMEM16f on platelet signaling, functional activity, and microparticle formation. TMEM16f-/- mice were obtained from Andrea Vortkamp and the platelets of these mice were isolated, stimulated with either dual agonist (thrombin + convulxin) or calcium ionophore (A23187), and platelet surface exposure of phosphatidylserine (PS) and microparticle generation were assessed by both flow cytometry and high resolution immunofluorescent confocal microscopy. To measure PS exposure, annexin V binding to platelet surfaces was quantified: In response to thrombin and convulxin, only 10.65% ± 1.35% of TMEM16f-/- platelets exposed PS after dual agonist exposure, compared to 16.1% ± 2.3%PS+ WT platelets when analyzed by high resolution microscopy. When analyzed by flow cytometry, dual agonist exposure of WT platelets yielded 52.4% ± 7.0% of maximal annexin V binding achieved by 10μM A23187; in contrast, there was no significant increase in annexin V binding detected in TMEM16f-/- platelets treated with dual agonist. Responses to calcium ionophore were also reduced in TMEM16f-/- platelets relative to WT (17.3% ± 8.3% PS+ TMEM16f-/- platelets in response to 1μM A23187 compared to 40% ± 1.4% PS+ WT platelets, by high resolution microscopy). Microparticle generation from TMEM16f-/- platelets compared to WT platelets was also evaluated by high resolution immunofluorescent microscopy and flow cytometry. On average, approximately 5.7 ± 0.33 microparticles were generated by untreated WT platelets however thrombin and convulxin treated platelets yielded 10.66 ± 0.92 microparticles per platelet whereas no significant increase in microparticle generation was observed in TMEM16f-/- platelets after dual agonist treatment. These data suggest that TMEM16f is required not only for PS exposure on platelet membranes, but also for the shedding of PS+ microparticles. TMEM16f-/- mice also have a significant defect in stable occlusive thrombus formation following 10% ferric chloride injury of the carotid artery (0/6 TMEM16f-/- mice form stable thrombi compared to 5/6 WT mice forming stable thrombi under the same conditions). TMEM16f-/- platelets had a slight (15%) reduction in total fibrinogen binding stimulated by PAR4 peptide agonist compared to WT, but showed no significant differences in aggregation to PAR4 agonist peptide or ADP compared to WT control platelets, suggesting that the defect in thrombus formation is likely due to PS- or microparticle-dependent procoagulant activity. This hypothesis is supported by preliminary results demonstrating that when microparticles isolated from ionophore-stimulated WT platelets were injected into TMEM16f knockout mice, 3/3 TMEM16f-/- mice formed stable thrombi, compared to 5/6 WT mice injected with vehicle control. In conclusion, TMEM16f-/- mice are deficient in platelet PS exposure, platelet-derived microparticle formation, and injury-induced thrombus formation, and platelet-derived microparticles appear to contribute to the defect in thrombus formation. Disclosures:No relevant conflicts of interest to declare.

A Role for Platelet TRPC Channels in the Ca 2+ Response That Induces Procoagulant Activity

After vascular injury, platelets are rapidly activated by collagen and other agonists, causing them to adhere and aggregate to prevent blood loss. In addition, phosphatidylserine (PS) exposure on the platelet surface accelerates thrombin formation by the coagulation pathway. Thrombin is a potent platelet agonist and converts fibrinogen to fibrin, thereby stabilizing the platelet plug. PS exposure during hemostasis and thrombosis results from a sustained cytosolic Ca(2+) increase; however, the underlying Ca(2+) mobilization pathways have remained unclear. Store-operated Orai1 channels provide substantial, prolonged Ca(2+) influx after inositol trisphosphate-dependent release, and anoctamin 6 (TMEM16F) may operate as a Ca(2+)-activated, Ca(2+)-permeable channel in addition to its scramblase activity that exteriorizes PS. A new study shows that Na(+) entry, resulting from coactivation of the transient receptor potential (TRP) nonselective cation channels TRPC3 and TRPC6, followed by reverse-mode operation of Na(+)/Ca(2+) exchangers, is an important mechanism for the increase in cytosolic Ca(2+) that triggers PS exposure, particularly during combined thrombin and collagen stimulation.