Integrins Form an Expanding Diffusional Barrier that Coordinates Phagocytosis.

Phagocytosis is initiated by lateral clustering of receptors, which in turn activates Src-family kinases (SFKs). Activation of SFKs requires depletion of tyrosine phosphatases from the area of particle engagement. We investigated how the major phosphatase CD45 is excluded from contact sites, using single-molecule tracking. The mobility of CD45 increased markedly upon engagement of Fcγ receptors. While individual CD45 molecules moved randomly, they were displaced from the advancing phagocytic cup by an expanding diffusional barrier. By micropatterning IgG, the ligand of Fcγ receptors, we found that the barrier extended well beyond the perimeter of the receptor-ligand engagement zone. Second messengers generated by Fcγ receptors activated integrins, which formed an actin-tethered diffusion barrier that excluded CD45. The expanding integrin wave facilitates the zippering of Fcγ receptors onto the target and integrates the information from sparse receptor-ligand complexes, coordinating the progression and ultimate closure of the phagocytic cup.

RAS P21 Protein Activator 3 (RASA3) Specifically Promotes Pathogenic T Helper 17 Cell Generation by Repressing T-Helper-2-Cell-Biased Programs.

Pathogenic Th17 (pTh17) cells drive inflammation and immune-pathology, but whether pTh17 cells are a Th17 cell subset whose generation is under specific molecular control remains unaddressed. We found that Ras p21 protein activator 3 (RASA3) was highly expressed by pTh17 cells relative to non-pTh17 cells and was required specifically for pTh17 generation in vitro and in vivo. Mice conditionally deficient for Rasa3 in T cells showed less pathology during experimental autoimmune encephalomyelitis. Rasa3-deficient T cells acquired a Th2 cell-biased program that dominantly trans-suppressed pTh17 cell generation via interleukin 4 production. The Th2 cell bias of Rasa3-deficient T cells was due to aberrantly elevated transcription factor IRF4 expression. RASA3 promoted proteasome-mediated IRF4 protein degradation by facilitating interaction of IRF4 with E3-ubiquitin ligase Cbl-b. Therefore, a RASA3-IRF4-Cbl-b pathway specifically directs pTh17 cell generation by balancing reciprocal Th17-Th2 cell programs. These findings indicate that a distinct molecular program directs pTh17 cell generation and reveals targets for treating pTh17 cell-related pathology and diseases.

4D intravital imaging identifies platelets as the predominant cellular procoagulant surface in a mouse hemostasis model.

Interplay between platelets, coagulation factors, endothelial cells (ECs) and fibrinolytic factors is necessary for effective hemostatic plug formation. This study describes a four-dimensional (4D) imaging platform to visualize and quantify hemostatic plug components in mice with high spatiotemporal resolution. Fibrin accumulation following laser-induced vascular injury was observed at the platelet plug-EC interface, controlled by the antagonistic balance between fibrin generation and breakdown. We observed less fibrin accumulation in mice expressing low levels of tissue factor (TFlow) or F12-/- mice compared to controls, whereas increased fibrin accumulation, including on the vasculature adjacent to the platelet plug, was observed in plasminogen-deficient mice or wild-type mice treated with tranexamic acid (TXA). Phosphatidylserine (PS), a membrane lipid critical for the assembly of coagulation factors, was first detected at the platelet plug-EC interface, followed by exposure across the endothelium. Impaired PS exposure resulted in a significant reduction in fibrin accumulation in cyclophilin D-/- mice. Adoptive transfer studies demonstrated a key role for PS exposure on platelets, and to a lesser degree on ECs, in fibrin accumulation during hemostatic plug formation. Together, these studies suggest that (1) platelets are the functionally dominant procoagulant cellular surface, and (2) plasmin is critical for limiting fibrin accumulation at the site of a forming hemostatic plug.

Mice expressing nonpolymerizable fibrinogen have reduced arterial and venous thrombosis with preserved hemostasis.

ABSTRACT: Elevated circulating fibrinogen levels correlate with increased risk for both cardiovascular and venous thromboembolic diseases. In vitro studies show that formation of a highly dense fibrin matrix is a major determinant of clot structure and stability. Here, we analyzed the impact of nonpolymerizable fibrinogen on arterial and venous thrombosis as well as hemostasis in vivo using FgaEK mice that express normal levels of a fibrinogen that cannot be cleaved by thrombin. In a model of carotid artery thrombosis, FgaWT/EK and FgaEK/EK mice were protected from occlusion with 4% ferric chloride (FeCl3) challenges compared with wild-type (FgaWT/WT) mice, but this protection was lost, with injuries driven by higher concentrations of FeCl3. In contrast, fibrinogen-deficient (Fga-/-) mice showed no evidence of occlusion, even with high-concentration FeCl3 challenge. Fibrinogen-dependent platelet aggregation and intraplatelet fibrinogen content were similar in FgaWT/WT, FgaWT/EK, and FgaEK/EK mice, consistent with preserved fibrinogen-platelet interactions that support arterial thrombosis with severe challenge. In an inferior vena cava stasis model of venous thrombosis, FgaEK/EK mice had near complete protection from thrombus formation. FgaWT/EK mice also displayed reduced thrombus incidence and a significant reduction in thrombus mass relative to FgaWT/WT mice after inferior vena cava stasis, suggesting that partial expression of nonpolymerizable fibrinogen was sufficient for conferring protection. Notably, FgaWT/EK and FgaEK/EK mice had preserved hemostasis in multiple models as well as normal wound healing times after skin incision, unlike Fga-/- mice that displayed significant bleeding and delayed healing. These findings indicate that a nonpolymerizable fibrinogen variant can significantly suppress occlusive thrombosis while preserving hemostatic potential in vivo.

Platelet biology and function: plaque erosion vs. rupture.

The leading cause of heart disease in developed countries is coronary atherosclerosis, which is not simply a result of ageing but a chronic inflammatory process that can lead to acute clinical events upon atherosclerotic plaque rupture or erosion and arterial thrombus formation. The composition and location of atherosclerotic plaques determine the phenotype of the lesion and whether it is more likely to rupture or to erode. Although plaque rupture and erosion both initiate platelet activation on the exposed vascular surface, the contribution of platelets to thrombus formation differs between the two phenotypes. In this review, plaque phenotype is discussed in relation to thrombus composition, and an overview of important mediators (haemodynamics, matrix components, and soluble factors) in plaque-induced platelet activation is given. As thrombus formation on disrupted plaques does not necessarily result in complete vessel occlusion, plaque healing can occur. Therefore, the latest findings on plaque healing and the potential role of platelets in this process are summarized. Finally, the clinical need for more effective antithrombotic agents is highlighted.

Fibrin(ogen) engagement of S. aureus promotes the host antimicrobial response and suppression of microbe dissemination following peritoneal infection.

The blood-clotting protein fibrin(ogen) plays a critical role in host defense against invading pathogens, particularly against peritoneal infection by the Gram-positive microbe Staphylococcus aureus. Here, we tested the hypothesis that direct binding between fibrin(ogen) and S. aureus is a component of the primary host antimicrobial response mechanism and prevention of secondary microbe dissemination from the peritoneal cavity. To establish a model system, we showed that fibrinogen isolated from FibγΔ5 mice, which express a mutant form lacking the final 5 amino acids of the fibrinogen γ chain (termed fibrinogenγΔ5), did not support S. aureus adherence when immobilized and clumping when in suspension. In contrast, purified wildtype fibrinogen supported robust adhesion and clumping that was largely dependent on S. aureus expression of the receptor clumping factor A (ClfA). Following peritoneal infection with S. aureus USA300, FibγΔ5 mice displayed worse survival compared to WT mice coupled to reduced bacterial killing within the peritoneal cavity and increased dissemination of the microbes into circulation and distant organs. The failure of acute bacterial killing, but not enhanced dissemination, was partially recapitulated by mice infected with S. aureus USA300 lacking ClfA. Fibrin polymer formation and coagulation transglutaminase Factor XIII each contributed to killing of the microbes within the peritoneal cavity, but only elimination of polymer formation enhanced systemic dissemination. Host macrophage depletion or selective elimination of the fibrin(ogen) ß2-integrin binding motif both compromised local bacterial killing and enhanced S. aureus systemic dissemination, suggesting fibrin polymer formation in and of itself was not sufficient to retain S. aureus within the peritoneal cavity. Collectively, these findings suggest that following peritoneal infection, the binding of S. aureus to stabilized fibrin matrices promotes a local, macrophage-mediated antimicrobial response essential for prevention of microbe dissemination and downstream host mortality.

Both G protein-coupled and immunoreceptor tyrosine-based activation motif receptors mediate venous thrombosis in mice.

Platelets are critical in hemostasis and a major contributor to arterial thrombosis (AT). (Pre)clinical studies suggest platelets also contribute to venous thrombosis (VT), but the mechanisms are largely unknown. We hypothesized that in VT, platelets use signaling machinery distinct from AT. Here we aimed to characterize the contributions of platelet G protein-coupled (GPCR) and immunoreceptor tyrosine-based activation motif (ITAM) receptor signaling to VT. Wild-type (WT) and transgenic mice were treated with inhibitors to selectively inhibit platelet-signaling pathways: ITAM-CLEC2 (Clec2mKO), glycoprotein VI (JAQ1 antibody), and Bruton's tyrosine kinase (ibrutinib); GPCR-cyclooxygenase 1 (aspirin); and P2Y12 (clopidogrel). VT was induced by inferior vena cava stenosis. Thrombin generation in platelet-rich plasma and whole-blood clot formation were studied ex vivo. Intravital microscopy was used to study platelet-leukocyte interactions after flow restriction. Thrombus weights were reduced in WT mice treated with high-dose aspirin + clopidogrel (dual antiplatelet therapy [DAPT]) but not in mice treated with either inhibitor alone or low-dose DAPT. Similarly, thrombus weights were reduced in mice with impaired ITAM signaling (Clec2mKO + JAQ1; WT + ibrutinib) but not in Clec2mKO or WT + JAQ1 mice. Both aspirin and clopidogrel, but not ibrutinib, protected mice from FeCl3-induced AT. Thrombin generation and clot formation were normal in blood from high-dose DAPT- or ibrutinib-treated mice; however, platelet adhesion and platelet-neutrophil aggregate formation at the vein wall were reduced in mice treated with high-dose DAPT or ibrutinib. In summary, VT initiation requires platelet activation via GPCRs and ITAM receptors. Strong inhibition of either signaling pathway reduces VT in mice.

Deletion of platelet CLEC-2 decreases GPIbα-mediated integrin αIIbß3 activation and decreases thrombosis in TTP.

Microvascular thrombosis in patients with thrombotic thrombocytopenic purpura (TTP) is initiated by GPIbα-mediated platelet binding to von Willebrand factor (VWF). Binding of VWF to GPIbα causes activation of the platelet surface integrin αIIbß3. However, the mechanism of GPIbα-initiated activation of αIIbß3 and its clinical importance for microvascular thrombosis remain elusive. Deletion of platelet C-type lectin-like receptor 2 (CLEC-2) did not prevent VWF binding to platelets but specifically inhibited platelet aggregation induced by VWF binding in mice. Deletion of platelet CLEC-2 also inhibited αIIbß3 activation induced by the binding of VWF to GPIbα. Using a mouse model of TTP, which was created by infusion of anti-mouse ADAMTS13 monoclonal antibodies followed by infusion of VWF, we found that deletion of platelet CLEC-2 decreased pulmonary arterial thrombosis and the severity of thrombocytopenia. Importantly, prophylactic oral administration of aspirin, an inhibitor of platelet activation, and therapeutic treatment of the TTP mice with eptifibatide, an integrin αIIbß3 antagonist, reduced pulmonary arterial thrombosis in the TTP mouse model. Our observations demonstrate that GPIbα-mediated activation of integrin αIIbß3 plays an important role in the formation of thrombosis in TTP. These observations suggest that prevention of platelet activation with aspirin may reduce the risk for thrombosis in patients with TTP.

Hypofibrinogenemia with preserved hemostasis and protection from thrombosis in mice with an Fga truncation mutation.

Genetic variants within the fibrinogen Aα chain encoding the αC-region commonly result in hypodysfibrinogenemia in patients. However, the (patho)physiological consequences and underlying mechanisms of such mutations remain undefined. Here, we generated Fga270 mice carrying a premature termination codon within the Fga gene at residue 271. The Fga270 mutation was compatible with Mendelian inheritance for offspring of heterozygous crosses. Adult Fga270/270 mice were hypofibrinogenemic with ∼10% plasma fibrinogen levels relative to FgaWT/WT mice, linked to 90% reduction in hepatic Fga messenger RNA (mRNA) because of nonsense-mediated decay of the mutant mRNA. Fga270/270 mice had preserved hemostatic potential in vitro and in vivo in models of tail bleeding and laser-induced saphenous vein injury, whereas Fga-/- mice had continuous bleeding. Platelets from FgaWT/WT and Fga270/270 mice displayed comparable initial aggregation following adenosine 5'-diphosphate stimulation, but Fga270/270 platelets quickly disaggregated. Despite ∼10% plasma fibrinogen, the fibrinogen level in Fga270/270 platelets was ∼30% of FgaWT/WT platelets with a compensatory increase in fibronectin. Notably, Fga270/270 mice showed complete protection from thrombosis in the inferior vena cava stasis model. In a model of Staphylococcus aureus peritonitis, Fga270/270 mice supported local, fibrinogen-mediated bacterial clearance and host survival comparable to FgaWT/WT, unlike Fga-/- mice. Decreasing the normal fibrinogen levels to ∼10% with small interfering RNA in mice also provided significant protection from venous thrombosis without compromising hemostatic potential and antimicrobial function. These findings both reveal novel molecular mechanisms underpinning fibrinogen αC-region truncation mutations and highlight the concept that selective fibrinogen reduction may be efficacious for limiting thrombosis while preserving hemostatic and immune protective functions.

Heightened activation of embryonic megakaryocytes causes aneurysms in the developing brain of mice lacking podoplanin.

During early embryonic development in mammals, including humans and mice, megakaryocytes (Mks) first originate from primitive hematopoiesis in the yolk sac. These embryonic Mks (eMks) circulate in the vasculature with unclear function. Herein, we report that podoplanin (PDPN), the ligand of C-type lectin-like receptor (CLEC-2) on Mks/platelets, is temporarily expressed in neural tissue during midgestation in mice. Loss of PDPN or CLEC-2 resulted in aneurysms and spontaneous hemorrhage, specifically in the lower diencephalon during midgestation. Surprisingly, more eMks/platelets had enhanced granule release and localized to the lower diencephalon in mutant mouse embryos than in wild-type littermates before hemorrhage. We found that PDPN counteracted the collagen-1-induced secretion of angiopoietin-1 from fetal Mks, which coincided with enhanced TIE-2 activation in aneurysm-like sprouts of PDPN-deficient embryos. Blocking platelet activation prevented the PDPN-deficient embryo from developing vascular defects. Our data reveal a new role for PDPN in regulating eMk function during midgestation.

The CalDAG-GEFI/Rap1/αIIbß3 axis minimally contributes to accelerated platelet clearance in mice with constitutive store-operated calcium entry.

Circulating platelets maintain low cytosolic Ca2+ concentrations. At sites of vascular injury, agonist-induced Ca2+ release from platelet intracellular stores triggers influx of extracellular Ca2+, a process known as store-operated Ca2+ entry (SOCE). Stromal interaction molecule 1 (Stim1) senses reduced Ca2+ stores and triggers SOCE. Gain-of-function (GOF) mutations in Stim1, such as described for Stormorken syndrome patients or mutant mice (Stim1Sax), are associated with marked thrombocytopenia and increased platelet turnover. We hypothesized that reduced platelet survival in Stim1Sax/+ mice is due to increased Rap1/integrin signaling and platelet clearance in the spleen, similar to what we recently described for mice expressing a mutant version of the Rap1-GAP, Rasa3 (Rasa3hlb/hlb). Stim1Sax/+ mice were crossed with mice deficient in CalDAG-GEFI, a critical calcium-regulated Rap1-GEF in platelets. In contrast to Rasa3hlb/hlb x Caldaggef1-/- mice, only a small increase in the peripheral platelet count, but not platelet lifespan, was observed in Stim1Sax/+ x Caldaggef1-/- mice. Similarly, inhibition of αIIbß3 integrin in vivo only minimally raised the peripheral platelet count in Stim1Sax/+ mice. Compared to controls, Stim1Sax/+ mice exhibited increased platelet accumulation in the lung, but not the spleen or liver. These results suggest that CalDAG-GEFI/Rap1/integrin signaling contributes only minimally to accelerated platelet turnover caused by constitutive SOCE.

What do we know? Platelets are small blood cells which act to prevent blood loss, which circulate in a resting state but are rapidly activated upon exposure to ligands at the site of vascular injuryCalcium (Ca²⁺) is critical for platelet activation, especially for activation of integrins which support platelet­platelet interactionsIf platelet activation occurs in circulation, platelets can be prematurely cleared from blood and unable to function in hemostasisDisorders of Ca²⁺ dysregulation such as Stormorken syndrome are associated with reduced platelet counts (thrombocytopenia) and bleedingWhat did we discover? We used a mouse model expressing a mutation causing higher Ca²⁺ levels in cells including platelets (Stim1^Sax), and investigated whether thrombocytopenia is due to stimulation of a specific pathway for integrin activation, mediated by a protein called Rap1 GTPaseWe crossed Stim1^Sax mice with mice lacking an important activator of Rap1, the Ca²⁺-regulated protein CalDAG-GEFI, and saw no major improvement in thrombocytopeniaWe also observed more Stim1^Sax platelets in the lung but not the liver or spleen, in contrast to mice with activation of platelet integrins in circulationWhat is the impact? Our results rule out activation of the CalDAG-GEFI/Rap1/integrin pathway as a major cause of thrombocytopenia in Stim1^Sax miceOur findings help to narrow down potential causes of thrombocytopenia in disorders such as Stormorken syndrome.

Talin-1 is the principal platelet Rap1 effector of integrin activation.

Ras-related protein 1 (Rap1) is a major convergence point of the platelet-signaling pathways that result in talin-1 binding to the integrin ß cytoplasmic domain and consequent integrin activation, platelet aggregation, and effective hemostasis. The nature of the connection between Rap1 and talin-1 in integrin activation is an important remaining gap in our understanding of this process. Previous work identified a low-affinity Rap1-binding site in the talin-1 F0 domain that makes a small contribution to integrin activation in platelets. We recently identified an additional Rap1-binding site in the talin-1 F1 domain that makes a greater contribution than F0 in model systems. Here we generated mice bearing point mutations, which block Rap1 binding without affecting talin-1 expression, in either the talin-1 F1 domain (R118E) alone, which were viable, or in both the F0 and F1 domains (R35E,R118E), which were embryonic lethal. Loss of the Rap1-talin-1 F1 interaction in platelets markedly decreases talin-1-mediated activation of platelet ß1- and ß3-integrins. Integrin activation and platelet aggregation in mice whose platelets express only talin-1(R35E, R118E) are even more impaired, resembling the defect seen in platelets lacking both Rap1a and Rap1b. Although Rap1 is important in thrombopoiesis, platelet secretion, and surface exposure of phosphatidylserine, loss of the Rap1-talin-1 interaction in talin-1(R35E, R118E) platelets had little effect on these processes. These findings show that talin-1 is the principal direct effector of Rap1 GTPases that regulates platelet integrin activation in hemostasis.

Red blood cells modulate structure and dynamics of venous clot formation in sickle cell disease.

Sickle cell disease (SCD) is associated with chronic activation of coagulation and an increased risk of venous thromboembolism. Erythrocyte sickling, the primary pathologic event in SCD, results in dramatic morphological changes in red blood cells (RBCs) because of polymerization of the abnormal hemoglobin. We used a mouse model of SCD and blood samples from sickle patients to determine if these changes affect the structure, properties, and dynamics of sickle clot formation. Sickling of RBCs and a significant increase in fibrin deposition were observed in venous thrombi formed in sickle mice. During ex vivo clot contraction, the number of RBCs extruded from sickle whole blood clots was significantly reduced compared with the number released from sickle cell trait and nonsickle clots in both mice and humans. Entrapment of sickled RBCs was largely factor XIIIa-independent and entirely mediated by the platelet-free cellular fraction of sickle blood. Inhibition of phosphatidylserine, but not administration of antisickling compounds, increased the number of RBCs released from sickle clots. Interestingly, whole blood, but not plasma clots from SCD patients, was more resistant to fibrinolysis, indicating that the cellular fraction of blood mediates resistance to tissue plasminogen activator. Sickle trait whole blood clots demonstrated an intermediate phenotype in response to tissue plasminogen activator. RBC exchange in SCD patients had a long-lasting effect on normalizing whole blood clot contraction. Furthermore, RBC exchange transiently reversed resistance of whole blood sickle clots to fibrinolysis, in part by decreasing platelet-derived PAI-1. These properties of sickle clots may explain the increased risk of venous thromboembolism observed in SCD.

Desialylation of O-glycans on glycoprotein Ibα drives receptor signaling and platelet clearance.

During infection neuraminidase desialylates platelets and induces their rapid clearance from circulation. The underlying molecular basis, particularly the role of platelet glycoprotein (GP)Ibα therein, is not clear. Utilizing genetically altered mice we report that the extracellular domain of GPIbα, but neither von Willebrand factor nor ADAM17 (a disintegrin and metalloprotease 17), is required for platelet clearance induced by intravenous injection of neuraminidase. Lectin binding to platelets following neuraminidase injection over time revealed that the extent of desialylation of O-glycans correlates with the decrease of platelet count in mice. Injection of α2,3-neuraminidase reduces platelet counts in wild-type but not in transgenic mice expressing only a chimeric GPIbα that misses most of its extracellular domain. Neuraminidase treatment induces unfolding of the O-glycosylated mechanosensory domain in GPIbα as monitored by single-molecule force spectroscopy, increases the exposure of the ADAM17 shedding cleavage site in the mechanosensory domain on the platelet surface, and induces ligand-independent GPIb-IX signaling in human and murine platelets. These results suggest that desialylation of O-glycans of GPIbα induces unfolding of the mechanosensory domain, subsequent GPIb-IX signaling including amplified desialylation of N-glycans, and eventually rapid platelet clearance. This new molecular mechanism of GPIbα-facilitated clearance could potentially resolve many puzzling and seemingly contradicting observations associated with clearance of desialylated or hyposialylated platelets.

Novel Mouse Model for Studying Hemostatic Function of Human Platelets.

OBJECTIVE: Platelets are critical to the formation of a hemostatic plug and the pathogenesis of atherothrombosis. Preclinical animal models, especially the mouse, provide an important platform to assess the efficacy and safety of antiplatelet drugs. However, these studies are limited by inherent differences between human and mouse platelets and the species-selectivity of many drugs. To circumvent these limitations, we developed a new protocol for the adoptive transfer of human platelets into thrombocytopenic nonobese diabetic/severe combined immune deficiency mice, that is, a model where all endogenous platelets are replaced by human platelets in mice accepting xenogeneic tissues. Approach and Results: To demonstrate the power of this new model, we visualized and quantified hemostatic plug formation and stability by intravital spinning disk confocal microscopy following laser ablation injury to the saphenous vein. Integrin αIIbß3-dependent hemostatic platelet plug formation was achieved within ≈30 seconds after laser ablation injury in humanized platelet mice. Pretreatment of mice with standard dual antiplatelet therapy (Aspirin+Ticagrelor) or PAR1 inhibitor, L-003959712 (an analog of vorapaxar), mildly prolonged the bleeding time and significantly reduced platelet adhesion to the site of injury. Consistent with findings from clinical trials, inhibition of PAR1 in combination with dual antiplatelet therapy markedly prolonged bleeding time in humanized platelet mice. CONCLUSIONS: We propose that this novel mouse model will provide a robust platform to test and predict the safety and efficacy of experimental antiplatelet drugs and to characterize the hemostatic function of synthetic, stored and patient platelets.

Platelet transfusion for patients with platelet dysfunction: effectiveness, mechanisms, and unanswered questions.

PURPOSE OF REVIEW: In this review, we discuss current clinical guidelines and potential underlying mechanisms regarding platelet transfusion therapy in patients at risk of bleeding, comparing management of patients with thrombocytopenia versus those with qualitative platelet disorders. RECENT FINDINGS: Platelet transfusion therapy is highly effective in managing bleeding in patients with hypoproliferative thrombocytopenia. Clinical trials have demonstrated that platelet transfusion can be used at a lower trigger threshold and reduced platelet doses, and may be used therapeutically rather than prophylactically in some situations, although additional data are needed. In patients with inherited platelet disorders such as Glanzmann's Thrombasthenia or those with RASGRP2 mutations, platelet transfusion may be ineffective because of competition between transfused and endogenous platelets at the site of vascular injury. Successful management of these patients may require transfusion of additional platelet units, or mechanism-driven combination therapy with other pro-hemostatic agents. In patients on antiplatelet therapy, timing of transfusion and inhibitor mechanism-of-action are key in determining therapeutic success. SUMMARY: Expanding our understanding of the mechanisms by which transfused platelets exert their pro-hemostatic function in various bleeding disorders will improve the appropriate use of platelet transfusion.

Functional redundancy between RAP1 isoforms in murine platelet production and function.

RAP GTPases, important regulators of cellular adhesion, are abundant signaling molecules in the platelet/megakaryocytic lineage. However, mice lacking the predominant isoform, RAP1B, display a partial platelet integrin activation defect and have a normal platelet count, suggesting the existence of a RAP1-independent pathway to integrin activation in platelets and a negligible role for RAP GTPases in megakaryocyte biology. To determine the importance of individual RAP isoforms on platelet production and on platelet activation at sites of mechanical injury or vascular leakage, we generated mice with megakaryocyte-specific deletion (mKO) of Rap1a and/or Rap1b Interestingly, Rap1a/b-mKO mice displayed a marked macrothrombocytopenia due to impaired proplatelet formation by megakaryocytes. In platelets, RAP isoforms had redundant and isoform-specific functions. Deletion of RAP1B, but not RAP1A, significantly reduced α-granule secretion and activation of the cytoskeleton regulator RAC1. Both isoforms significantly contributed to thromboxane A2 generation and the inside-out activation of platelet integrins. Combined deficiency of RAP1A and RAP1B markedly impaired platelet aggregation, spreading, and clot retraction. Consistently, thrombus formation in physiological flow conditions was abolished in Rap1a/b-mKO, but not Rap1a-mKO or Rap1b-mKO, platelets. Rap1a/b-mKO mice were strongly protected from experimental thrombosis and exhibited a severe defect in hemostasis after mechanical injury. Surprisingly, Rap1a/b-mKO platelets were indistinguishable from controls in their ability to prevent blood-lymphatic mixing during development and hemorrhage at sites of inflammation. In summary, our studies demonstrate an essential role for RAP1 signaling in platelet integrin activation and a critical role in platelet production. Although important for hemostatic/thrombotic plug formation, platelet RAP1 signaling is dispensable for vascular integrity during development and inflammation.

Glanzmann thrombasthenia: genetic basis and clinical correlates.

Glanzmann thrombasthenia (GT) is an autosomal recessive disorder of platelet aggregation caused by quantitative or qualitative defects in integrins αIIb and ß3. These integrins are encoded by the ITGA2B and ITGB3 genes and form platelet glycoprotein (GP)IIb/IIIa, which acts as the principal platelet receptor for fibrinogen. Although there is variability in the clinical phenotype, most patients present with severe mucocutaneous bleeding at an early age. A classic pattern of abnormal platelet aggregation, platelet glycoprotein expression and molecular studies confirm the diagnosis. Management of bleeding is based on a combination of hemostatic agents including recombinant activated factor VII with or without platelet transfusions and antifibrinolytic agents. Refractory bleeding and platelet alloimmunization are common complications. In addition, pregnant patients pose unique management challenges. This review highlights clinical and molecular aspects in the approach to patients with GT, with particular emphasis on the significance of multidisciplinary care.

Calcium-induced structural rearrangements release autoinhibition in the Rap-GEF CalDAG-GEFI.

Platelets are recruited to sites of vascular injury, where they are activated and aggregate to form a hemostatic plug. This process requires the activation of the small GTPase Rap1B by its cognate guanine nucleotide exchange factor CalDAG-GEFI. Studies on platelet function suggest that CalDAG-GEFI activity is regulated by changes in cytosolic calcium, but the exact molecular mechanism is poorly understood. Here we show that purified CalDAG-GEFI is autoinhibited and directly regulated by calcium. Substitutions of putative calcium-binding residues within the canonical EF hands of CalDAG-GEFI diminish its capacity to activate Rap1B. Structural differences between active (WT) and inactive (EF hand variant) CalDAG-GEFI protein were determined by hydrogen-deuterium exchange MS. The highest differential rates of deuterium uptake in WT over EF hand variant CalDAG-GEFI were observed in regions within the catalytic Cdc25 domain and a putative autoinhibitory linker connecting the Cdc25 and EF hand domains. Exchange activity in the EF hand variant was fully restored by an additional substitution, valine 406 to glutamate, which is thought to disrupt the interface between the autoinhibitory linker and the Cdc25 domain. Overall, our results suggest a model for how CalDAG-GEFI remains in an autoinhibited state when levels of cytosolic calcium in resting platelets are low. In response to cellular stimulation, calcium mobilization and binding to the EF hands causes conformational rearrangements within CalDAG-GEFI, including the autoinhibitory linker that frees the catalytic surface of CalDAG-GEFI to engage and activate Rap1B. The data from this study are the first evidence linking CalDAG-GEFI activity directly to calcium.

Anticoagulant Protein S Targets the Factor IXa Heparin-Binding Exosite to Prevent Thrombosis.

OBJECTIVE: PS (protein S) is a plasma protein that directly inhibits the coagulation FIXa (factor IXa) in vitro. Because elevated FIXa is associated with increased risk of venous thromboembolism, it is important to establish how PS inhibits FIXa function in vivo. The goal of this study is to confirm direct binding of PS with FIXa in vivo, identify FIXa amino acid residues required for binding PS in vivo, and use an enzymatically active FIXa mutant that is unable to bind PS to measure the significance of PS-FIXa interaction in hemostasis. APPROACH AND RESULTS: We demonstrate that PS inhibits FIXa in vivo by associating with the FIXa heparin-binding exosite. We used fluorescence tagging, immunohistochemistry, and protein-protein crosslinking to show in vivo interaction between FIXa and PS. Importantly, platelet colocalization required a direct interaction between the 2 proteins. FIXa and PS also coimmunoprecipitated from plasma, substantiating their interaction in a physiological milieu. PS binding to FIXa and PS inhibition of the intrinsic Xase complex required residues K132, K126, and R170 in the FIXa heparin-binding exosite. A double mutant, K132A/R170A, retained full activity but could not bind to PS. Crucially, Hemophilia B mice infused with FIXa K132A/R170A displayed an accelerated rate of fibrin clot formation compared with wild-type FIXa. CONCLUSIONS: Our findings establish PS as an important in vivo inhibitor of FIXa. Disruption of the interaction between PS and FIXa causes an increased rate of thrombus formation in mice. This newly discovered function of PS implies an unexploited target for antithrombotic therapeutics.