Frauke May

Publications of Frauke May

• 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.

• CLEC-2 is an essential platelet activating receptor in hemostasis and thrombosis.

  Authors: Frauke May, Ina Hagedorn, Irina Pleines, Markus Bender, Timo Vogtle, Johannes Eble, Margitta Elvers, Bernhard Nieswandt

  Blood. 08/2009;

  Damage to the integrity of the vessel wall leads to exposure of the subendothelial extracellular matrix (ECM) which triggers platelet activation and aggregation. This process is essential for primaryDamage to the integrity of the vessel wall leads to exposure of the subendothelial extracellular matrix (ECM) which triggers platelet activation and aggregation. This process is essential for primary hemostasis but it may also lead to arterial thrombosis. While the mechanisms underlying platelet activation on the ECM are well explored, it is less clear which receptors mediate cellular activation in a growing thrombus. Here we studied the role of the recently identified C-type lectin-like receptor 2 (CLEC-2) in this process. We show that anti-CLEC-2 antibody treatment of mice leads to complete and highly specific loss of CLEC-2 in circulating platelets for several days. CLEC-2-deficient platelets displayed normal adhesion under flow but subsequent aggregate formation was severely defective in vitro and in vivo. As a consequence, CLEC-2 deficiency was associated with increased bleeding times and profound protection from occlusive arterial thrombus formation. These results reveal an essential function of CLEC-2 in hemostasis and thrombosis.

• Rac1 is essential for phospholipase C-gamma2 activation in platelets.

  Authors: Irina Pleines, Margitta Elvers, Amrei Strehl, Miroslava Pozgajova, David Varga-Szabo, Frauke May, Anna Chrostek-Grashoff, Cord Brakebusch, Bernhard Nieswandt

  Pflugers Archiv : European journal of physiology. 09/2008;

  Platelet activation at sites of vascular injury is triggered through different signaling pathways leading to activation of phospholipase (PL) Cbeta or PLCgamma2. Active PLCs trigger Ca(2+)Platelet activation at sites of vascular injury is triggered through different signaling pathways leading to activation of phospholipase (PL) Cbeta or PLCgamma2. Active PLCs trigger Ca(2+) mobilization and entry, which is a prerequisite for adhesion, secretion, and thrombus formation. PLCbeta isoenzymes are activated downstream of G protein-coupled receptors (GPCRs), whereas PLCgamma2 is activated downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors, such as the major platelet collagen receptor glycoprotein (GP) VI or CLEC-2. The mechanisms underlying PLC regulation are not fully understood. An involvement of small GTPases of the Rho family (Rho, Rac, Cdc42) in PLC activation has been proposed but this has not been investigated in platelets. We here show that murine platelets lacking Rac1 display severely impaired GPVI- or CLEC-2-dependent activation and aggregation. This defect was associated with impaired production of inositol 1,4,5-trisphosphate (IP(3)) and intracellular calcium mobilization suggesting inappropriate activation of PLCgamma2 despite normal tyrosine phosphorylation of the enzyme. Rac1 ( -/- ) platelets displayed defective thrombus formation on collagen under flow conditions which could be fully restored by co-infusion of ADP and the TxA(2) analog U46619, indicating that impaired GPVI-, but not G-protein signaling, was responsible for the observed defect. In line with this, Rac1 ( -/- ) mice were protected in two collagen-dependent arterial thrombosis models. Together, these results demonstrate that Rac1 is essential for ITAM-dependent PLCgamma2 activation in platelets and that this is critical for thrombus formation in vivo.