Bernhard Nieswandt

Publications of Bernhard Nieswandt

• Antithrombotic Potential of Blockers of Store-Operated Calcium Channels in Platelets.

  Authors: Roger van Kruchten, Attila Braun, Marion A H Feijge, Marijke J E Kuijpers, Ronmy Rivera-Galdos, Peter Kraft, Guido Stoll, Christoph Kleinschnitz, Edouard M Bevers, Bernhard Nieswandt, Johan W M Heemskerk

  Arteriosclerosis, thrombosis, and vascular biology. 05/2012;

  OBJECTIVE: Platelet Orai1 channels mediate store-operated Ca(2+) entry (SOCE), which is required for procoagulant activity and arterial thrombus formation. Pharmacological blockage of these channelsOBJECTIVE: Platelet Orai1 channels mediate store-operated Ca(2+) entry (SOCE), which is required for procoagulant activity and arterial thrombus formation. Pharmacological blockage of these channels may provide a novel way of antithrombotic therapy. Therefore, the thromboprotective effect of SOCE blockers directed against platelet Orai1 is determined. METHODS AND RESULTS: Candidate inhibitors were screened for their effects on SOCE in washed human platelets. Tested antagonists included the known compounds, SKF96365, 2-aminoethyl diphenylborate, and MRS1845 and the novel compounds, Synta66 and GSK-7975A. The potency of SOCE inhibition was in the order of Synta66>2-aminoethyl diphenylborate>GSK-7975A>SKF96365>MRS1845. The specificity of the first 3 compounds was verified with platelets from Orai1-deficient mice. Inhibitory activity on procoagulant activity and high-shear thrombus formation was assessed in plasma and whole blood. In the presence of plasma, all 3 compounds suppressed platelet responses and restrained thrombus formation under flow. Using a murine stroke model, arterial thrombus formation was provoked in vivo by transient middle cerebral artery occlusion. Postoperative administration of 2-aminoethyl diphenylborate markedly diminished brain infarct size. CONCLUSIONS: Plasma-soluble SOCE blockers such as 2-aminoethyl diphenylborate suppress platelet-dependent coagulation and thrombus formation. The platelet Orai1 channel is a novel target for preventing thrombotic events causing brain infarction.

• Focal cerebral ischemia.

  Authors: Stefan Braeuninger, Christoph Kleinschnitz, Bernhard Nieswandt, Guido Stoll

  Methods in molecular biology (Clifton, N.J.). 01/2012; 788:29-42.

  Rodent models of focal cerebral ischemia have been extremely useful in elucidating pathomechanisms of human stroke. Most commonly, a monofilament is advanced through the internal carotid artery ofRodent models of focal cerebral ischemia have been extremely useful in elucidating pathomechanisms of human stroke. Most commonly, a monofilament is advanced through the internal carotid artery of rodents to occlude the origin of the middle cerebral artery thus leading to critical ischemia in the corresponding vascular territory. The filament can be removed after different occlusion times allowing reperfusion (transient middle cerebral artery occlusion (MCAO) model) or is left permanently within the internal carotid artery (permanent MCAO model) both mimicking clinical thromboembolic stroke in which the occluding clot may resolve spontaneously or after thrombolysis, or may persist. Overall, the occlusion time determines the extent of ischemic brain damage, but infarcts still grow during reperfusion, a process involving complex interactions between platelets, endothelial cells, immune cells, and the coagulation system.

• A platelet-mediated system for shuttling blood-borne bacteria to CD8α+ dendritic cells depends on glycoprotein GPIb and complement C3.

  Authors: Admar Verschoor, Michael Neuenhahn, Alexander A Navarini, Patricia Graef, Ann Plaumann, Amelie Seidlmeier, Bernhard Nieswandt, Steffen Massberg, Rolf M Zinkernagel, Hans Hengartner, Dirk H Busch

  Nature immunology. 12/2011; 12(12):1194-201.

  The acquisition of pathogen-derived antigen by dendritic cells (DCs) is a key event in the generation of cytotoxic CD8(+) T cell responses. In mice, the intracellular bacterium Listeria monocytogenesThe acquisition of pathogen-derived antigen by dendritic cells (DCs) is a key event in the generation of cytotoxic CD8(+) T cell responses. In mice, the intracellular bacterium Listeria monocytogenes is directed from the blood to splenic CD8α(+) DCs. We report that L. monocytogenes rapidly associated with platelets in the bloodstream in a manner dependent on GPIb and complement C3. Platelet association targeted a small but immunologically important portion of L. monocytogenes to splenic CD8α(+) DCs, diverting bacteria from swift clearance by other, less immunogenic phagocytes. Thus, an effective balance is established between maintaining sterility of the circulation and induction of antibacterial immunity by DCs. Other gram-positive bacteria also were rapidly tagged by platelets, revealing a broadly active shuttling mechanism for systemic bacteria.

• Megakaryocyte-specific RhoA deficiency causes macrothrombocytopenia and defective platelet activation in hemostasis and thrombosis.

  Authors: Irina Pleines, Ina Hagedorn, Shuchi Gupta, Frauke May, Lidija Chakarova, Jolanda van Hengel, Stefan Offermanns, Georg Krohne, Christoph Kleinschnitz, Cord Brakebusch, Bernhard Nieswandt

  Blood. 11/2011; 119(4):1054-63.

  Vascular injury initiates rapid platelet activation that is critical for hemostasis, but it also may cause thrombotic diseases, such as myocardial infarction or ischemic stroke. Reorganizations ofVascular injury initiates rapid platelet activation that is critical for hemostasis, but it also may cause thrombotic diseases, such as myocardial infarction or ischemic stroke. Reorganizations of the platelet cytoskeleton are crucial for platelet shape change and secretion and are thought to involve activation of the small GTPase RhoA. In this study, we analyzed the in vitro and in vivo consequences of megakaryocyte- and platelet-specific RhoA gene deletion in mice. We found a pronounced macrothrombocytopenia in RhoA-deficient mice, with platelet counts of approximately half that of wild-type controls. The mutant cells displayed an altered shape but only a moderately reduced life span. Shape change of RhoA-deficient platelets in response to G(13)-coupled agonists was abolished, and it was impaired in response to G(q) stimulation. Similarly, RhoA was required for efficient secretion of α and dense granules downstream of G(13) and G(q). Furthermore, RhoA was essential for integrin-mediated clot retraction but not for actomyosin rearrangements and spreading of activated platelets on fibrinogen. In vivo, RhoA deficiency resulted in markedly prolonged tail bleeding times but also significant protection in different models of arterial thrombosis and in a model of ischemic stroke. Together, these results establish RhoA as an important regulator of platelet function in thrombosis and hemostasis.

• Blood coagulation factor XII--a neglected player in stroke pathophysiology.

  Authors: Mirko Pham, Guido Stoll, Bernhard Nieswandt, Martin Bendszus, Christoph Kleinschnitz

  Journal of molecular medicine (Berlin, Germany). 09/2011; 90(2):119-26.

  Ischemic stroke is a devastating disease which, in most cases, is caused by thrombotic occlusion of brain arteries. The molecular mechanisms involved in microvascular thrombus formation during focalIschemic stroke is a devastating disease which, in most cases, is caused by thrombotic occlusion of brain arteries. The molecular mechanisms involved in microvascular thrombus formation during focal cerebral ischemia are not well understood. As a consequence, the current antithrombotic drugs used to treat acute stroke or prevent stroke recurrence either show limited efficacy or put patients at risk for serious bleeding complications. The serine protease blood coagulation factor XII (FXII) initiates the intrinsic pathway of coagulation which, together with the extrinsic pathway, culminates in the formation of fibrin. A physiological function of FXII in clot formation and hemostasis in vivo has been questioned for more than 50 years. This was mainly due to the fact that hereditary FXII deficiency does not induce any bleeding phenotype in humans. However, recent studies in transgenic mice challenged this concept by demonstrating that FXII deficiency prevents pathological thrombus formation, but does not affect regular hemostasis. These findings entailed investigations in relevant disease models of thromboembolism including ischemic stroke. The present review summarizes the pathophysiological role of FXII in experimental cerebral ischemia and highlights novel therapeutic strategies based on FXII inhibition.

• Critical role for stromal interaction molecule 1 in cardiac hypertrophy.

  Authors: Jean-Sébastien Hulot, Jérémy Fauconnier, Deepak Ramanujam, Antoine Chaanine, Fleur Aubart, Yassine Sassi, Sabine Merkle, Olivier Cazorla, Aude Ouillé, Morgan Dupuis [......] Joachim Schmitt, Attila Braun, Ludovic Bénard, Youakim Saliba, Bernhard Laggerbauer, Bernhard Nieswandt, Alain Lacampagne, Roger J Hajjar, Anne-Marie Lompré, Stefan Engelhardt

  Circulation. 08/2011; 124(7):796-805.

  Cardiomyocytes use Ca2+ not only in excitation-contraction coupling but also as a signaling molecule promoting, for example, cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling inCardiomyocytes use Ca2+ not only in excitation-contraction coupling but also as a signaling molecule promoting, for example, cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in cardiomyocytes in the presence of the rapid and large Ca2+ fluctuations that occur during excitation-contraction coupling. A potential route is store-operated Ca2+ entry, a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). Store-operated Ca2+ entry can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+ entry with respect to cardiac hypertrophy in vitro and in vivo. Consistent with earlier reports, we found drug-inducible store-operated Ca2+ entry in neonatal rat cardiomyocytes, which was dependent on STIM1. Although this STIM1-dependent, drug-inducible store-operated Ca2+ entry was only marginal in adult cardiomyocytes isolated from control hearts, it increased significantly in cardiomyocytes isolated from adult rats that had developed compensated cardiac hypertrophy after abdominal aortic banding. Moreover, we detected an inwardly rectifying current in hypertrophic cardiomyocytes that occurs under native conditions (i.e., in the absence of drug-induced store depletion) and is dependent on STIM1. By manipulating its expression, we found STIM1 to be both sufficient and necessary for cardiomyocyte hypertrophy in vitro and in the adult heart in vivo. Stim1 silencing by adeno-associated viruses of serotype 9-mediated gene transfer protected rats from pressure overload-induced cardiac hypertrophy. By controlling a previously unrecognized sarcolemmal current, STIM1 promotes cardiac hypertrophy.

• Ischaemic stroke: a thrombo-inflammatory disease?

  Authors: Bernhard Nieswandt, Christoph Kleinschnitz, Guido Stoll

  The Journal of physiology. 07/2011; 589(Pt 17):4115-23.

  Ischaemic stroke is a leading cause of death and disability worldwide. The complex cellular interactions leading from thromboembolic vessel occlusion to infarct development within the brainIschaemic stroke is a leading cause of death and disability worldwide. The complex cellular interactions leading from thromboembolic vessel occlusion to infarct development within the brain parenchyma in acute stroke are poorly understood, which translates into only one approved effective treatment, thrombolysis. Importantly, however, patients can develop progressive stroke despite reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury' which can be reproduced in the mouse model of transient middle cerebral artery occlusion (tMCAO). Although pathological platelet and coagulant activity have long been recognized to be involved in the initiation of ischaemic stroke, their contribution to infarct maturation remained elusive. Experimental evidence now suggests that early platelet adhesion/activation mechanisms involving the von Willebrand factor (vWF) receptor glycoprotein (GP) Ib, its ligand vWF, and the collagen receptor GPVI are critical pathogenic factors in infarct development following tMCAO, whereas platelet aggregation through GPIIb/IIIa is not. Further experimental work indicates that these pathways in conjunction with coagulation factor XII (FXII)-driven processes orchestrate a 'thrombo-inflammatory' cascade in acute stroke that results in infarct growth. This review summarizes these recent developments and briefly discusses their potential clinical impact.

• STIM and Orai in platelet function.

  Authors: David Varga-Szabo, Attila Braun, Bernhard Nieswandt

  Cell calcium. 05/2011; 50(3):270-8.

  Physiological platelet activation and thrombus formation are essential to stop bleeding in case of vascular injury, whereas inadequate triggering of the same process in diseased vessels can lead toPhysiological platelet activation and thrombus formation are essential to stop bleeding in case of vascular injury, whereas inadequate triggering of the same process in diseased vessels can lead to fatal thromboembolism and tissue ischemia of vital organs. A central step in platelet activation is agonist-induced elevation of the intracellular Ca(2+) concentration. This happens on the one hand through the release of Ca(2+) from intracellular stores and on the other hand through Ca(2+) influx from the extracellular space. In platelets, the major Ca(2+) influx pathway is the so-called store operated Ca(2+) entry (SOCE), induced by store depletion. Studies in the last five years discovered the molecular background of platelet SOCE. Stromal interaction molecule 1 (STIM1) and Orai1, two so far unknown molecules, got in the focus of research. STIM1 was found to be the Ca(2+) sensor in the endoplasmic reticulum (ER) membrane, whereas Orai1 was identified as the major store operated Ca(2+) (SOC) channel in the plasma membrane. These two molecules and their role in platelet function and thrombus formation are the topic of the present review with a special focus on apoptosis and apoptosis-like processes in platelet physiology.

• Dividing VI by X(a).

  Authors: Bernhard Nieswandt, Johan W M Heemskerk

  Blood. 04/2011; 117(14):3704-5.

• Sugar rush bleeds the brain.

  Authors: Bernhard Nieswandt, Guido Stoll

  Nature medicine. 02/2011; 17(2):161-2.

• STIM and Orai in hemostasis and thrombosis.

  Authors: Attila Braun, Timo Vogtle, David Varga-Szabo, Bernhard Nieswandt

  Frontiers in bioscience : a journal and virtual library. 01/2011; 17:2144-60.

  At sites of vascular injury, platelets rapidly adhere to the exposed subendothelial extracellular matrix, become activated and, together with the coagulation system, form a plug that seals theAt sites of vascular injury, platelets rapidly adhere to the exposed subendothelial extracellular matrix, become activated and, together with the coagulation system, form a plug that seals the lesion. This process is essential to prevent blood loss, however, under pathological conditions it may lead to vessel occlusion. Agonist-induced elevation of intracellular Ca(2+) levels is essential for platelet activation. It occurs through two different mechanisms: Ca(2+) release from internal stores, involving phospholipase C (PLC)-dependent generation of inositol-1,4,5-trisphosphate (IP3) and activation of IP3 sensitive channels in the store membrane, and Ca(2+) influx across the plasma membrane. Store operated Ca(2+) entry (SOCE), triggered by store depletion, is the main influx pathway for extracellular Ca(2+) in platelets, but the molecular mechanism underlying this pathway has long remained elusive. In the last years, however, the Ca(2+) sensor stromal interaction molecule 1 (STIM1) and the channel protein Orai1 emerged as the key players in platelet SOCE. This review summarizes the current knowledge about the role of these proteins in platelet physiology and thrombus formation and discusses their suitability as antithrombotic targets.

• Sustained reperfusion after blockade of glycoprotein-receptor-Ib in focal cerebral ischemia: an MRI study at 17.6 Tesla.

  Authors: Mirko Pham, Xavier Helluy, Christoph Kleinschnitz, Peter Kraft, Andreas J Bartsch, Peter Jakob, Bernhard Nieswandt, Martin Bendszus, Guido Stoll

  PloS one. 01/2011; 6(4):e18386.

  Inhibition of early platelet adhesion by blockade of glycoprotein-IB (GPIb) protects mice from ischemic stroke. To elucidate underlying mechanisms in-vivo, infarct development was followed byInhibition of early platelet adhesion by blockade of glycoprotein-IB (GPIb) protects mice from ischemic stroke. To elucidate underlying mechanisms in-vivo, infarct development was followed by ultra-high field MRI at 17.6 Tesla. Cerebral infarction was induced by transient-middle-cerebral-artery-occlusion (tMCAO) for 1 hour in C57/BL6 control mice (N = 10) and mice treated with 100 µg Fab-fragments of the GPIb blocking antibody p0p/B 1 h after tMCAO (N = 10). To control for the effect of reperfusion, additional mice underwent permanent occlusion and received anti-GPIb treatment (N = 6; pMCAO) or remained without treatment (N = 3; pMCAO). MRI 2 h and 24 h after MCAO measured cerebral-blood-flow (CBF) by continuous arterial-spin labelling, the apparent-diffusion-coefficient (ADC), quantitative-T2 and T2-weighted imaging. All images were registered to a standard mouse brain MRI atlas and statistically analysed voxel-wise, and by cortico-subcortical ROI analysis. Anti-GPIb treatment led to a relative increase of postischemic CBF vs. controls in the cortical territory of the MCA (2 h: 44.2±6.9 ml/100 g/min versus 24 h: 60.5±8.4; p = 0.0012, F((1,18)) = 14.63) after tMCAO. Subcortical CBF 2 h after tMCAO was higher in anti-GPIb treated animals (45.3±5.9 vs. controls: 33.6±4.3; p = 0.04). In both regions, CBF findings were clearly related to a lower probability of infarction (Cortex/Subcortex of treated group: 35%/65% vs. controls: 95%/100%) and improved quantitative-T2 and ADC. After pMCAO, anti-GPIb treated mice developed similar infarcts preceded by severe irreversible hypoperfusion as controls after tMCAO indicating dependency of stroke protection on reperfusion. Blockade of platelet adhesion by anti-GPIb-Fab-fragments results in substantially improved CBF early during reperfusion. This finding was in exact spatial correspondence with the prevention of cerebral infarction and indicates in-vivo an increased patency of the microcirculation. Thus, progression of infarction during early ischemia and reperfusion can be mitigated by anti-platelet treatment.

• Platelet receptor signaling in thrombus formation.

  Authors: David Stegner, Bernhard Nieswandt

  Journal of molecular medicine (Berlin, Germany). 11/2010; 89(2):109-21.

  Platelet activation and subsequent thrombus formation at sites of vascular injury is crucial for normal hemostasis, but it can also cause myocardial infarction and stroke. The initial capture ofPlatelet activation and subsequent thrombus formation at sites of vascular injury is crucial for normal hemostasis, but it can also cause myocardial infarction and stroke. The initial capture of flowing platelets to the injured vessel wall is mediated by the interaction of the glycoprotein (GP) Ib-V-IX complex with von Willebrand factor immobilized on the exposed subendothelial extracellular matrix. Tethered platelets are then able to bind to collagens through the immunoglobulin-like receptor GPVI and to initiate cellular activation, a process that is reinforced by G protein-coupled receptors stimulated by locally produced thrombin and soluble mediators released from activated platelets. These signaling events lead to a rise in the cytosolic Ca(2+) concentration, rearrangement of the cytoskeleton, release of granule content, and functional upregulation of integrin adhesion receptors allowing firm adhesion and thrombus growth. Fully activated platelets also undergo a procoagulant conversion thereby facilitating coagulation and thrombus stabilization. This review summarizes the most important receptor systems and signaling mechanisms involved in platelet activation and thrombus formation with special focus on recent discoveries.

• Differentially regulated GPVI ectodomain shedding by multiple platelet-expressed proteinases.

  Authors: Markus Bender, Sebastian Hofmann, David Stegner, Athena Chalaris, Michael Bösl, Attila Braun, Jürgen Scheller, Stefan Rose-John, Bernhard Nieswandt

  Blood. 10/2010; 116(17):3347-55.

  Glycoprotein VI (GPVI) mediates platelet activation on exposed subendothelial collagens at sites of vascular injury and thereby contributes to normal hemostasis, but also to the occlusion of diseasedGlycoprotein VI (GPVI) mediates platelet activation on exposed subendothelial collagens at sites of vascular injury and thereby contributes to normal hemostasis, but also to the occlusion of diseased vessels in the setting of myocardial infarction or stroke. GPVI is an attractive target for antithrombotic therapy, particularly because previous studies have shown that anti-GPVI antibodies induce irreversible down-regulation of the receptor in circulating platelets by internalization and/or ectodomain shedding. Metalloproteinases of the a disintegrin and metalloproteinase (ADAM) family have been proposed to mediate this ectodomain shedding, but direct evidence for this is lacking. Here, we studied GPVI shedding in vitro and in vivo in newly generated mice with a megakaryocyte-specific ADAM10 deficiency and in Adam17(ex/ex) mice, which lack functional ADAM17. We demonstrate that GPVI cleavage in vitro can occur independently through either ADAM10 or ADAM17 in response to distinct stimuli. In contrast, antibody (JAQ1)-induced GPVI shedding in vivo occurred in mice lacking both ADAM10/ADAM17 in their platelets, suggesting the existence of a third GPVI cleaving platelet enzyme. This was supported by in vitro studies on ADAM10/ADAM17 double-deficient platelets. These results reveal that ectodomain shedding of GPVI can be mediated through multiple differentially regulated platelet-expressed proteinases with obvious therapeutic implications.

• Binding of von Willebrand factor to collagen and glycoprotein Ibalpha, but not to glycoprotein IIb/IIIa, contributes to ischemic stroke in mice--brief report.

  Authors: Simon F De Meyer, Tobias Schwarz, Hans Deckmyn, Cécile V Denis, Bernhard Nieswandt, Guido Stoll, Karen Vanhoorelbeke, Christoph Kleinschnitz

  Arteriosclerosis, thrombosis, and vascular biology. 10/2010; 30(10):1949-51.

  To unravel crucial von Willebrand factor (VWF) interactions that are detrimental in stroke development. VWF(-/-) mice received gene transfer to express mutants of VWF defective either in binding toTo unravel crucial von Willebrand factor (VWF) interactions that are detrimental in stroke development. VWF(-/-) mice received gene transfer to express mutants of VWF defective either in binding to fibrillar collagen, glycoprotein (GP)Ibα or GPIIb/IIIa, and underwent 60 minutes of transient middle cerebral artery occlusion. In VWF(-/-) mice reconstituted with VWF mutants defective in binding to collagen or GPIbα, protection against stroke was sustained, whereas VWF lacking the GPIIb/IIIa binding site restored full susceptibility similar to normal VWF. VWF-collagen and VWF-GPIbα (but not VWF-GPIIb/IIIa) interactions are instrumental in thrombus formation after transient middle cerebral artery occlusion, and their inhibition could be a promising target for stroke treatment.

• Combating innate inflammation: a new paradigm for acute treatment of stroke?

  Authors: Guido Stoll, Christoph Kleinschnitz, Bernhard Nieswandt

  Annals of the New York Academy of Sciences. 10/2010; 1207:149-54.

  Interference with early steps of platelet adhesion/activation by inhibition of the von Willebrand factor (vWF) receptor glycoprotein (GP)Ib, its ligand vWF, or the collagen receptor GPVI, profoundlyInterference with early steps of platelet adhesion/activation by inhibition of the von Willebrand factor (vWF) receptor glycoprotein (GP)Ib, its ligand vWF, or the collagen receptor GPVI, profoundly limits infarction in the mouse stroke model of transient middle cerebral artery occlusion (tMCAO). A similar pathogenic role was revealed for coagulation factor XII (FXII). Although these findings strongly suggest that microvascular thrombus formation is the leading pathophysiological event in acute stroke, recent studies have shown that these molecules have the additional capacity to guide inflammatory processes, thereby providing an intriguing alternative mechanistic explanation for these observations. Surprisingly, mice lacking T cells are also protected from acute stroke, and these T cell effects are antigen independent. Thus, acute ischemic stroke can be redefined as a thrombo-inflammatory disorder, and multifunctional molecules such as GPIb, GPVI, and FXII may provide new therapeutic targets linking inflammation and thrombus formation.

• ADF/n-cofilin-dependent actin turnover determines platelet formation and sizing.

  Authors: Markus Bender, Anita Eckly, John H Hartwig, Margitta Elvers, Irina Pleines, Shuchi Gupta, Georg Krohne, Elisabeth Jeanclos, Antje Gohla, Christine Gurniak, Christian Gachet, Walter Witke, Bernhard Nieswandt

  Blood. 09/2010; 116(10):1767-75.

  The cellular and molecular mechanisms orchestrating the complex process by which bone marrow megakaryocytes form and release platelets remain poorly understood. Mature megakaryocytes generate longThe cellular and molecular mechanisms orchestrating the complex process by which bone marrow megakaryocytes form and release platelets remain poorly understood. Mature megakaryocytes generate long cytoplasmic extensions, proplatelets, which have the capacity to generate platelets. Although microtubules are the main structural component of proplatelets and microtubule sliding is known to drive proplatelet elongation, the role of actin dynamics in the process of platelet formation has remained elusive. Here, we tailored a mouse model lacking all ADF/n-cofilin-mediated actin dynamics in megakaryocytes to specifically elucidate the role of actin filament turnover in platelet formation. We demonstrate, for the first time, that in vivo actin filament turnover plays a critical role in the late stages of platelet formation from megakaryocytes and the proper sizing of platelets in the periphery. Our results provide the genetic proof that platelet production from megakaryocytes strictly requires dynamic changes in the actin cytoskeleton.

• Impact of glycoprotein VI and platelet adhesion on atherosclerosis--a possible role of fibronectin.

  Authors: Andreas Bültmann, Zhongmin Li, Silvia Wagner, Mario Peluso, Tanja Schönberger, Carla Weis, Ildiko Konrad, Konstantinos Stellos, Steffen Massberg, Bernhard Nieswandt, Meinrad Gawaz, Martin Ungerer, Götz Münch

  Journal of molecular and cellular cardiology. 09/2010; 49(3):532-42.

  Glycoprotein VI (GPVI) mediates binding of platelets to subendothelial collagen during acute arterial thrombosis. GPVI interactions with the activated atherosclerotic vascular endothelium duringGlycoprotein VI (GPVI) mediates binding of platelets to subendothelial collagen during acute arterial thrombosis. GPVI interactions with the activated atherosclerotic vascular endothelium during early atherosclerosis, however, are not well understood. In ApoE-/- mice, platelet adhesion to atherosclerotic arteries was increased, as measured by intravital microscopy. This platelet adhesion was significantly inhibited by IV injection of GPVI-Fc (1 mg/kg body weight). Atherosclerosis in ApoE-/- mice was attenuated both after 7 and 10 weeks of treatment with the anti-GPVI antibody JAQ1 (2 mg/kg body weight i.p. twice weekly). Binding of GPVI-Fc (1 mg/kg IV) occurred to deeper layers, but also to the luminal site of plaques in atherosclerotic rabbits, but not to the vessel wall of healthy littermates. Gene transfer of GPVI-Fc to the carotid vascular wall significantly attenuated athero-progression and endothelial dysfunction in atherosclerotic rabbits in vivo. Specific binding of the soluble GPVI receptor (GPVI-Fc) to fibronectin was found in vitro to coated ELISA plates. Platelet adhesion to fibronectin was significantly inhibited both by GPVI-Fc and by the anti-GPVI antibody 5C4 ex vivo in flow chamber experiments. GPVI plays a role in platelet adhesion to atherosclerotic endothelium in the absence of plaque rupture. Inhibition of GPVI both via GPVI-Fc and anti-GPVI-antibodies results in protection against atherosclerosis in both cholesterol-fed rabbits and ApoE-/- mice. This novel mechanism of GPVI-mediated platelet adhesion-possibly via fibronectin-could relevantly contribute to platelet-triggered atheroprogression.

• Roles of platelet STIM1 and Orai1 in glycoprotein VI- and thrombin-dependent procoagulant activity and thrombus formation.

  Authors: Karen Gilio, Roger van Kruchten, Attila Braun, Alejandro Berna-Erro, Marion A H Feijge, David Stegner, Paola E J van der Meijden, Marijke J E Kuijpers, David Varga-Szabo, Johan W M Heemskerk, Bernhard Nieswandt

  The Journal of biological chemistry. 07/2010; 285(31):23629-38.

  In platelets, STIM1 has been recognized as the key regulatory protein in store-operated Ca(2+) entry (SOCE) with Orai1 as principal Ca(2+) entry channel. Both proteins contribute toIn platelets, STIM1 has been recognized as the key regulatory protein in store-operated Ca(2+) entry (SOCE) with Orai1 as principal Ca(2+) entry channel. Both proteins contribute to collagen-dependent arterial thrombosis in mice in vivo. It is unclear whether STIM2 is involved. A key platelet response relying on Ca(2+) entry is the surface exposure of phosphatidylserine (PS), which accomplishes platelet procoagulant activity. We studied this response in mouse platelets deficient in STIM1, STIM2, or Orai1. Upon high shear flow of blood over collagen, Stim1(-/-) and Orai1(-/-) platelets had greatly impaired glycoprotein (GP) VI-dependent Ca(2+) signals, and they were deficient in PS exposure and thrombus formation. In contrast, Stim2(-/-) platelets reacted normally. Upon blood flow in the presence of thrombin generation and coagulation, Ca(2+) signals of Stim1(-/-) and Orai1(-/-) platelets were partly reduced, whereas the PS exposure and formation of fibrin-rich thrombi were normalized. Washed Stim1(-/-) and Orai1(-/-) platelets were deficient in GPVI-induced PS exposure and prothrombinase activity, but not when thrombin was present as co-agonist. Markedly, SKF96365, a blocker of (receptor-operated) Ca(2+) entry, inhibited Ca(2+) and procoagulant responses even in Stim1(-/-) and Orai1(-/-) platelets. These data show for the first time that: (i) STIM1 and Orai1 jointly contribute to GPVI-induced SOCE, procoagulant activity, and thrombus formation; (ii) a compensating Ca(2+) entry pathway is effective in the additional presence of thrombin; (iii) platelets contain two mechanisms of Ca(2+) entry and PS exposure, only one relying on STIM1-Orai1 interaction.

• Early detrimental T-cell effects in experimental cerebral ischemia are neither related to adaptive immunity nor thrombus formation.

  Authors: Christoph Kleinschnitz, Nicholas Schwab, Peter Kraft, Ina Hagedorn, Angela Dreykluft, Tobias Schwarz, Madeleine Austinat, Bernhard Nieswandt, Heinz Wiendl, Guido Stoll

  Blood. 03/2010; 115(18):3835-42.

  T cells contribute to the pathophysiology of ischemic stroke by yet unknown mechanisms. Mice with transgenic T-cell receptors (TCRs) and mutations in costimulatory molecules were used to define theT cells contribute to the pathophysiology of ischemic stroke by yet unknown mechanisms. Mice with transgenic T-cell receptors (TCRs) and mutations in costimulatory molecules were used to define the minimal immunologic requirements for T cell-mediated ischemic brain damage. Stroke was induced in recombination activating gene 1-deficient (RAG1(-/-)) mice devoid of T and B cells, RAG1(-/-) mice reconstituted with B cells or T cells, TCR-transgenic mice bearing 1 single CD8(+) (2C/RAG2, OTI/RAG1 mice) or CD4(+) (OTII/RAG1, 2D2/RAG1 mice) TCR, mice lacking accessory molecules of TCR stimulation (CD28(-/-), PD1(-/-), B7-H1(-/-) mice), or mice deficient in nonclassical T cells (natural killer T [NKT] and gammadelta T cells) by transient middle cerebral artery occlusion (tMCAO). Stroke outcome was assessed at day 1. RAG1(-/-) mice and RAG1(-/-) mice reconstituted with B cells developed significantly smaller brain infarctions compared with controls, but thrombus formation after FeCl(3)-induced vessel injury was unimpaired. In contrast, TCR-transgenic mice and mice lacking costimulatory TCR signals were fully susceptible to tMCAO similar to mice lacking NKT and gammadelta T cells. These findings were corroborated by adoptive transfer experiments. Our data demonstrate that T cells critically contribute to cerebral ischemia, but their detrimental effect neither depends on antigen recognition nor TCR costimulation or thrombus formation.