Thrombosis and inflammation are intricately linked in several major clinical disorders, including disseminated intravascular coagulation and acute ischemic events. The damage-associated molecular pattern molecule high-mobility group box 1 (HMGB1) is upregulated by activated platelets in multiple inflammatory diseases; however, the contribution of platelet-derived HMGB1 in thrombosis remains unexplored. Here, we generated transgenic mice with platelet-specific ablation of HMGB1 and determined that platelet-derived HMGB1 is a critical mediator of thrombosis. Mice lacking HMGB1 in platelets exhibited increased bleeding times as well as reduced thrombus formation, platelet aggregation, inflammation, and organ damage during experimental trauma/hemorrhagic shock. Platelets were the major source of HMGB1 within thrombi. In trauma patients, HMGB1 expression on the surface of circulating platelets was markedly upregulated. Moreover, evaluation of isolated platelets revealed that HMGB1 is critical for regulating platelet activation, granule secretion, adhesion, and spreading. These effects were mediated via TLR4- and MyD88-dependent recruitment of platelet guanylyl cyclase (GC) toward the plasma membrane, followed by MyD88/GC complex formation and activation of the cGMP-dependent protein kinase I (cGKI). Thus, we establish platelet-derived HMGB1 as an important mediator of thrombosis and identify a HMGB1-driven link between MyD88 and GC/cGKI in platelets. Additionally, these findings suggest a potential therapeutic target for patients sustaining trauma and other inflammatory disorders associated with abnormal coagulation.

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Suicidal death of erythrocytes, or eryptosis, is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Eryptosis is triggered by increase of cytosolic Ca2+activity, which may result from treatment with the Ca2+ionophore ionomycin or from energy depletion by removal of glucose. The present study tested the hypothesis that phosphatidylserine exposure at the erythrocyte surface fosters adherence to endothelial cells of the vascular wall under flow conditions at arterial shear rates and that binding of eryptotic cells to endothelial cells is mediated by the transmembrane CXC chemokine ligand 16 (CXCL16). To this end, human erythrocytes were exposed to energy depletion (for 48 h) or treated with the Ca2+ionophore ionomycin (1 μM for 30 min). Phosphatidylserine exposure was quantified utilizing annexin-V binding, cell volume was estimated from forward scatter in FACS analysis, and erythrocyte adhesion to human vascular endothelial cells (HUVEC) was determined in a flow chamber model. As a result, both, ionomycin and glucose depletion, triggered eryptosis and enhanced the percentage of erythrocytes adhering to HUVEC under flow conditions at arterial shear rates. The adhesion was significantly blunted in the presence of erythrocyte phosphatidylserine-coating annexin-V (5 μl/ml), of a neutralizing antibody against endothelial CXCL16 (4 μg/ml), and following silencing of endothelial CXCL16 with small interfering RNA. The present observations demonstrate that eryptotic erythrocytes adhere to endothelial cells of the vascular wall in part by interaction of phosphatidylserine exposed at the erythrocyte surface with endothelial CXCL16.

IntroductionPlatelet adhesion, activation, and aggregation are essential for primary hemostasis at sites of vascular injury but are also critically important for the development of acute thrombotic occlusion at regions of atherosclerotic plaque rupture, the major pathophysiologic mechanism underlying myocardial infarction and ischemic stroke. 1 Platelet activation is triggered by various agonists, including subendothelial collagen, ADP released from activated platelets, thrombin generated by the coagulation cascade, or the collagen receptor glycoprotein VI (GPVI)-specific agonists convulxin (CVX) and collagen-related peptide (CRP). 2 The agonists lead to platelet granule release, integrin ␣ IIb ␤ 3 activation, phosphatidylserine exposure, aggregation, and thrombus formation. 2 All those platelet responses depend on an increase of cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] i ), 3,4 which is accomplished by inositol-1,4,5-triphosphatemediated Ca 2ϩ release from intracellular stores triggering subsequent stimulation of store-operated Ca 2ϩ entry (SOCE) across the plasma membrane. 5 Two key players in platelet SOCE have recently been identified: The 4-transmembrane-spanning poreforming calcium release-activated channel moiety Orai1, which mediates entry of extracellular Ca 2ϩ , and stromal interaction molecule 1 (STIM1), an Orai1 regulating Ca 2ϩ sensor expressed predominantly in the endoplasmic reticulum. [6][7][8] Regulators of Orai1 in other cell types include receptor for activated protein kinase C-1, 9 reactive oxygen species, 10 and lipid rafts. 11 However, regulation of Orai1 in platelets is poorly understood. Platelet activation has been shown to be regulated in vitro and in vivo by the PI3K/Akt signaling cascade. 12,13 Interference with PI3K signaling has previously been shown to compromise Ca 2ϩ influx into platelets. 14,15 Signaling molecules regulated by PI3K signaling include the serum-and glucocorticoid-inducible kinase 1 (SGK1), a kinase belonging to the AGC family of serine/threonine protein kinases. 16,17 SGK1 has originally been cloned as a glucocorticoidsensitive gene but later shown to be regulated by a variety of hormones and other triggers, including thrombin, growth factors IGF-1 and TGF-␤, oxidative stress, and ischemia. 17 SGK1 has previously been reported to regulate a wide variety of carriers and ion channels, including the epithelial Ca 2ϩ channels TRPV5 and TRPV6. 17 Most recently, SGK1 has been shown to be critically important for the Ca 2ϩ entry into mast cells after activation of the IgE receptor, 18 an effect mediated by regulation of Orai1. 19 Furthermore, SGK1 participates in the regulation of renal tubular Na ϩ reabsorption, salt appetite, and thus blood pressure. 17 A gain-of-function SGK1 gene variant, the combined presence of single nucleotide polymorphism in intron 6 (rs1743966) and in exon 8 (rs1057293), is associated with enhanced blood pressure. 20 Submitted June 9, 2011; accepted August 28, 2011. Prepublished online as Blood First Edition paper, October 26, 2011; DOI 10.1182...

Rationale: Macrophage migration inhibitory factor (MIF) is released on platelet activation. Circulating MIF could potentially regulate platelets and thereby platelet-mediated inflammatory and regenerative mechanisms. However, the effect of MIF on platelets is unknown. Objective: The present study evaluated MIF in regulating platelet survival and thrombotic potential. Methods and Results: MIF interacted with CXCR4-CXCR7 on platelets, defining CXCR7 as a hitherto unrecognized receptor for MIF on platelets. MIF internalized CXCR4, but unlike CXCL12 (SDF-1α), it did not phosphorylate Erk1/2 after CXCR4 ligation because of the lack of CD74 and failed in subsequent CXCR7 externalization. MIF did not alter the activation status of platelets. However, MIF rescued platelets from activation and BH3 mimetic ABT-737–induced apoptosis in vitro via CXCR7 and enhanced circulating platelet survival when administered in vivo. The antiapoptotic effect of MIF was absent in Cxcr7 −/− murine embryonic cells but pronounced in CXCR7-transfected Madin–Darby canine kidney cells. This prosurvival effect was attributed to the MIF–CXCR7–initiated PI3K-Akt pathway. MIF induced CXCR7-Akt–dependent phosphorylation of BCL-2 antagonist of cell death (BAD) both in vitro and in vivo. Consequentially, MIF failed to rescue Akt −/− platelets from thrombin-induced apoptosis when challenged ex vivo, also in prolonging platelet survival and in inducing BAD phosphorylation among Akt −/− mice in vivo. MIF reduced thrombus formation under arterial flow conditions in vitro and retarded thrombotic occlusion after FeCl 3 -induced arterial injury in vivo, an effect mediated through CXCR7. Conclusion: MIF interaction with CXCR7 modulates platelet survival and thrombotic potential both in vitro and in vivo and thus could regulate thrombosis and inflammation.