Platelet CD36 Signaling Through ERK5 Promotes Caspase-Dependent Procoagulant Activity and Fibrin Deposition In Vivo

Dyslipidemia is a risk factor for clinically significant thrombotic events. In this condition, scavenger receptor CD36 potentiates platelet reactivity through recognition of circulating oxidized lipids. CD36 promotes thrombosis by activating redox-sensitive signaling molecules, such as the MAPK extracellular signal-regulated kinase 5 (ERK5). However, the events downstream of platelet ERK5 are not clear. In this study, we report that oxidized low-density lipoprotein (oxLDL) promotes exposure of procoagulant phosphatidylserine (PSer) on platelet surfaces. Studies using pharmacologic inhibitors indicate that oxLDL-CD36 interaction–induced PSer exposure requires apoptotic caspases in addition to the downstream CD36-signaling molecules Src kinases, hydrogen peroxide, and ERK5. Caspases promote PSer exposure and, subsequently, recruitment of the prothrombinase complex, resulting in the generation of fibrin from the activation of thrombin. Caspase activity was observed when platelets were stimulated with oxLDL. This was prevented by inhibiting CD36 and ERK5. Furthermore, oxLDL potentiates convulxin/glycoprotein VI–mediated fibrin formation by platelets, which was prevented when CD36, ERK5, and caspases were inhibited. Using 2 in vivo arterial thrombosis models in apoE-null hyperlipidemic mice demonstrated enhanced arterial fibrin accumulation upon vessel injury. Importantly, absence of ERK5 in platelets or mice lacking CD36 displayed decreased fibrin accumulation in high-fat diet–fed conditions comparable to that seen in chow diet–fed animals. These findings suggest that platelet signaling through CD36 and ERK5 induces a procoagulant phenotype in the hyperlipidemic environment by enhancing caspase-mediated PSer exposure

A microengineered vascularized bleeding model that integrates the principal components of hemostasis

Hemostasis encompasses an ensemble of interactions among platelets, coagulation factors, blood cells, endothelium, and hemodynamic forces, but current assays assess only isolated aspects of this complex process. Accordingly, here we develop a comprehensive in vitro mechanical injury bleeding model comprising an “endothelialized” microfluidic system coupled with a microengineered pneumatic valve that induces a vascular “injury”. With perfusion of whole blood, hemostatic plug formation is visualized and “in vitro bleeding time” is measured. We investigate the interaction of different components of hemostasis, gaining insight into several unresolved hematologic issues. Specifically, we visualize and quantitatively demonstrate: the effect of anti-platelet agent on clot contraction and hemostatic plug formation, that von Willebrand factor is essential for hemostasis at high shear, that hemophilia A blood confers unstable hemostatic plug formation and altered fibrin architecture, and the importance of endothelial phosphatidylserine in hemostasis. These results establish the versatility and clinical utility of our microfluidic bleeding model

The Cardiac Tissue-Restricted Homeobox Protein Csx/Nkx2.5 Physically Associates with the Zinc Finger Protein GATA4 and Cooperatively Activates Atrial Natriuretic Factor Gene Expression

Specification and differentiation of the cardiac muscle lineage appear to require a combinatorial network of many factors. The cardiac muscle-restricted homeobox protein Csx/Nkx2.5 (Csx) is expressed in the precardiac mesoderm as well as the embryonic and adult heart. Targeted disruption of Csx causes embryonic lethality due to abnormal heart morphogenesis. The zinc finger transcription factor GATA4 is also expressed in the heart and has been shown to be essential for heart tube formation. GATA4 is known to activate many cardiac tissue-restricted genes. In this study, we tested whether Csx and GATA4 physically associate and cooperatively activate transcription of a target gene. Coimmunoprecipitation experiments demonstrate that Csx and GATA4 associate intracellularly. Interestingly, in vitro protein-protein interaction studies indicate that helix III of the homeodomain of Csx is required to interact with GATA4 and that the carboxy-terminal zinc finger of GATA4 is necessary to associate with Csx. Both regions are known to directly contact the cognate DNA sequences. The promoter-enhancer region of the atrial natriuretic factor (ANF) contains several putative Csx binding sites and consensus GATA4 binding sites. Transient-transfection assays indicate that Csx can activate ANF reporter gene expression to the same extent that GATA4 does in a DNA binding site-dependent manner. Coexpression of Csx and GATA4 synergistically activates ANF reporter gene expression. Mutational analyses suggest that this synergy requires both factors to fully retain their transcriptional activities, including the cofactor binding activity. These results demonstrate the first example of homeoprotein and zinc finger protein interaction in vertebrates to cooperatively regulate target gene expression. Such synergistic interaction among tissue-restricted transcription factors may be an important mechanism to reinforce tissue-specific developmental pathways

Dual Mechanism of Integrin alpha(IIb)beta(3) Closure in Procoagulant Platelets

Aggregation of platelets via activated integrin α(IIb)β(3) is a prerequisite for thrombus formation. Phosphatidylserine-exposing platelets with a key role in the coagulation process disconnect from a thrombus by integrin inactivation via an unknown mechanism. Here we show that α(IIb)β(3) inactivation in procoagulant platelets relies on a sustained high intracellular Ca(2+), stimulating intracellular cleavage of the β(3) chain, talin, and Src kinase. Inhibition of calpain activity abolished protein cleavage, but only partly suppressed α(IIb)β(3) inactivation. Integrin α(IIb)β(3) inactivation was unchanged in platelets from Capn1(−/−) mice, suggesting a role of the calpain-2 isoform. Scott syndrome platelets, lacking the transmembrane protein TMEM16F and having low phosphatidylserine exposure, displayed reduced α(IIb)β(3) inactivation with the remaining activity fully dependent on calpain. In platelets from Ppif(−/−) mice, lacking mitochondrial permeability transition pore (mPTP) formation, agonist-induced phosphatidylserine exposure and α(IIb)β(3) inactivation were reduced. Treatment of human platelets with cyclosporin A gave a similar phenotype. Together, these data point to a dual mechanism of α(IIb)β(3) inactivation via calpain(-2) cleavage of integrin-associated proteins and via TMEM16F-dependent phospholipid scrambling with an assistant role of mPTP formation

Von Willebrand Factor for Menorrhagia: A Survey and Literature Review

Abstract Background: von Willebrand disease (VWD) is the most common congenital bleeding disorder. In affected women, menorrhagia is the most common bleeding symptom. Combined oral contraceptives (COCs), the first choice therapy recommended by NHLBI 2007 guidelines, reduce menstrual loss by increasing coagulation factor levels, but at least 30% are unresponsive or intolerant. Non-hormonal options include the antifibrinolytic tranexamic acid (TA), reduces menstrual bleeding by 30-50%, but requires dosing three times a day. Intranasal desmopressin (Stimate) is simpler to use, but ineffective in ~20%. Effective treatment for menorrhagia, thus, remains the greatest unmet health need in women with VWD. Von Willebrand factor (VWF) is used typically when first-line and second-line treatments fail, but few data exist regarding its effectiveness in reducing menorrhagia. Methods: We therefore conducted a survey of U.S. hemophilia treatment centers (HTCs) of current therapy for menorrhagia in VWD, utilizing Centers for Disease Control (CDC) website https://www2a.cdc.gov/ncbddd/htcweb/Main.asp, and the Hemostasis and Research Society (HTRS) site http://htrs.org. To specifically assess the use of VWF concentrate for menorrhagia, we also performed a literature review using medical subject heading (MeSH) search terms "von Willebrand factor," "menorrhagia," and "von Willebrand disease." Statistical analysis was by descriptive statistics. Results: Of 83 surveys distributed to hemophilia treatment centers (HTCs) caring for adult patients, 35 HTC MDs responded (42.2%) but only 20 HTC MDs (24.1%) provided sufficient data for analysis. These 20 HTC MDs reported a total of 1,321 women with VWD age 18-45 years seen during the 3-year period 2011-2014, of whom 816 (61.8%) had menorrhagia. Among these women, the most common first-line therapy was COCs, reported by 50.0% of the 20 HTC MDs, TA in 30.0% and desmopressin (DDAVP) in 20.0%. Overall, including all therapies (first-, second-, third-line), DDAVP was prescribed by 90.0% of the 20 HTC MDs, TA in 80.0%, COCs in 70.0%, aminocaproic acid (amicar) in 25.0%, and the levonorgestrel-releasing intrauterine system (Mirena IUD) in 15.0%. Only 4 HTC MDs (20.0%) prescribed VWF concentrate (VWF) for menorrhagia: all used VWF as third-line therapy after first-line and second-line treatments had failed. In the 13 women with type 1, 2, or 3 VWD and menorrhagia treated with intravenous VWF by these 4 HTC MDs, there was reduction in menorrhagia in all 13 (100%), with no adverse effects. These patients learned intravenous technique and infused VWF at 40-50 IU/kg at home for up to 5 days of menstrual bleeding each cycle, with good acceptability. In the literature search, we identified six published studies, including two prospective clinical trials, two retrospective observational trials, and two observational network studies. A total of 455 subjects with VWD reported in these six studies were treated with either plasma-derived (pdVWF) or recombinant (rVWF) VWF for bleeds. Of these, nearly one-third or 138 (30.3%) were women with type 1, 2, or 3 VWD and menorrhagia who were treated with pdVWF or rVWF at a dose of 36-50 IU/kg for 1-6 days of menstrual cycle bleeding. In these studies, 95-100% of these women reported reduction in menorrhagia, with no reported adverse events. Discussion: This survey and literature review of 151 women with VWD and menorrhagia represent the largest treatment experience to date. DDAVP, TA, and COCs are the most common first-line therapies. VWF is a third-line therapy but safely and effectively reduces menorrhagia in at least 95% of women with VWD. Prospective clinical trials of VWF are needed to establish the minimal dose required for menorrhagia, to determine patient acceptability of this intravenous therapy, and to compare safety and efficacy with standard therapy. Disclosures Off Label Use: recombinant VWF and plasma-derived VWF for treatment of menorrhagia in VWD. Ragni:Bristol Myers Squibb: Research Funding; Biogen: Research Funding; Bayer: Research Funding; Baxalta: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Alnylam: Research Funding; Dimension Therapeutics: Research Funding; CSL Behring: Research Funding; Foundation Women Girls Blood Disorders: Membership on an entity's Board of Directors or advisory committees; Genentech Roche: Research Funding; Medscape, Web MD: Honoraria; National Hemophilia Foundation: Membership on an entity's Board of Directors or advisory committees; Pfizer: Research Funding; SPARK: Research Funding; Shire: Membership on an entity's Board of Directors or advisory committees, Research Funding; Tacere Benitec: Membership on an entity's Board of Directors or advisory committees; Ferring Pharmceuticals: Research Funding; Biomarin: Research Funding; Vascular Medicine Institute: Research Funding. Malec:Baxalta: Research Funding; Biogen: Research Funding. Coyle:Bayer: Membership on an entity's Board of Directors or advisory committees. Drygalski:Biogen: Consultancy; Baxalta: Consultancy; Novo Nordisk: Consultancy; Pfizer: Consultancy; Bayer: Consultancy; Hematherix Inc: Equity Ownership; Biogen: Research Funding; Baxalta: Research Funding; Novo Nordisk: Research Funding; Bayer: Research Funding; CSL Behring: Speakers Bureau; Hematherix Inc: Membership on an entity's Board of Directors or advisory committees. James:CSL Behring: Membership on an entity's Board of Directors or advisory committees; Baxter: Membership on an entity's Board of Directors or advisory committees. Jobe:Biogen: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; CSL-Behring: Membership on an entity's Board of Directors or advisory committees. Konkle:Octapharma: Research Funding; Baxalta: Consultancy; CSL Behring: Consultancy; Baxalta: Research Funding. Kouides:CSL Behring: Membership on an entity's Board of Directors or advisory committees. Kuriakose:Kedrion: Speakers Bureau. Ma:Baxalta: Membership on an entity's Board of Directors or advisory committees, Research Funding; Biogen Idec: Membership on an entity's Board of Directors or advisory committees, Research Funding; Kedrion: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees. Nance:Patient Services, Inc.: Membership on an entity's Board of Directors or advisory committees. Neff:Alexion: Membership on an entity's Board of Directors or advisory committees; Novonordisk: Research Funding; Baxter: Membership on an entity's Board of Directors or advisory committees; Kedrion: Membership on an entity's Board of Directors or advisory committees; Novonordisk: Membership on an entity's Board of Directors or advisory committees. Philipp:Baxter: Research Funding. Yaish:Agios: Membership on an entity's Board of Directors or advisory committees

Critical role for the mitochondrial permeability transition pore and cyclophilin D in platelet activation and thrombosis

Many of the cellular responses that occur in activated platelets resemble events that take place following activation of cell-death pathways in nucleated cells. We tested the hypothesis that formation of the mitochondrial permeability transition pore (MPTP), a key signaling event during cell death, also plays a critical role in platelet activation. Stimulation of murine platelets with thrombin plus the glycoprotein VI agonist convulxin resulted in a rapid loss of mitochondrial transmembrane potential (Δψm) in a subpopulation of activated platelets. In the absence of cyclophilin D (CypD), an essential regulator of MPTP formation, murine platelet activation responses were altered. CypD-deficient platelets exhibited defects in phosphatidylserine externalization, high-level surface fibrinogen retention, membrane vesiculation, and procoagulant activity. Also, in CypD-deficient platelet-rich plasma, clot retraction was altered. Stimulation with thrombin plus H2O2, a known activator of MPTP formation, also increased high-level surface fibrinogen retention, phosphatidylserine externalization, and platelet procoagulant activity in a CypD-dependent manner. In a model of carotid artery photochemical injury, thrombosis was markedly accelerated in CypD-deficient mice. These results implicate CypD and the MPTP as critical regulators of platelet activation and suggest a novel CypD-dependent negative-feedback mechanism regulating arterial thrombosis