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Background/Aims: The polyamine oxidase inhibitor MDL-72527 (N1,N4-bis(2,3-butadienyl)-1,4-butanediamine) were expected to increase the abundance of spermine, a powerful inhibitor of platelet activation. Nothing is known, however, on the sensitivity of platelet function and survival to MDL-72527 exposure. The present study thus explored whether MDL-72527 modifies function and survival of platelets without and with platelet activation by collagen related peptide (CRP). Methods: Platelets isolated from wild-type mice were exposed for 30 minutes to MDL-72527 (100 µM) with or without subsequent activation with CRP (2-5 µg/ml). Flow cytometry was employed to estimate cytosolic Ca2+-activity ([Ca2+]i) from Fluo-3 fluorescence, platelet degranulation from P-selectin abundance, integrin activation from αIIbβ3 integrin abundance, generation of reactive oxygen species (ROS) from DCFDA fluorescence, phospholipid scrambling of the cell membrane from annexin-V-binding, platelet volume from forward scatter and aggregation utilizing staining with CD9-APC and CD9-PE. Results: In the absence of CRP, exposure of platelets to MDL-72527 did not significantly modify [Ca2+]i, P-selectin abundance, αIIbβ3 integrin abundance, ROS, annexin-V-binding, and forward scatter. The addition of 2-5 µg/ml CRP was followed by significant increase of [Ca2+]i, P-selectin abundance, αIIbβ3 integrin activation, ROS abundance, annexin-V-binding, and aggregation as well as a significant decrease of forward scatter, all effects significantly blunted or virtually abolished in the presence of MDL-72527. Conclusions: MDL-72527 is a powerful inhibitor of platelet activation, apoptosis and aggregation.

Background/Aims: DAPT (24-diamino-5-phenylthiazole) inhibits γ-secretase, which cleaves the signaling molecule CD44, a negative regulator of platelet activation and apoptosis. CD44 is a co-receptor for macrophage migration inhibitory factor (MIF) an anti-apoptotic pro-inflammatory cytokine expressed and released from blood platelets. Whether DAPT influences platelet function, remained, however, elusive. Activators of platelets include collagen related peptide (CRP). The present study thus explored whether DAPT modifies the stimulating effect of CRP on platelet function. Methods: Platelets isolated from wild-type mice were exposed for 30 minutes to DAPT (10 µM). Flow cytometry was employed to estimate Orai1 abundance with specific antibodies, cytosolic Ca2+-activity ([Ca2+]i) from Fluo-3 fluorescence, platelet degranulation from P-selectin abundance, integrin activation from αIIbβ3 integrin abundance, generation of reactive oxygen species (ROS) from DCFDA fluorescence, mitochondrial transmembrane potential from TMRE fluorescence, phospholipid scrambling of the cell membrane from annexin-V-binding, relative platelet volume from forward scatter and aggregation utilizing staining with CD9-APC and CD9-PE. Results: Exposure of platelets to 2-5 µg/ml CRP was followed by significant increase of Orai1 abundance, [Ca2+]i, and P-selectin abundance, as well as by αIIbβ3 integrin activation, ROS generation, mitochondrial depolarization, enhanced annexin-V-binding, decreased cell volume, and aggregation. All CRP induced effects were significantly blunted in the presence of DAPT. Conclusions: The γ-secretase inhibitor DAPT counteracts agonist induced platelet activation, apoptosis and aggregation.

Background/Aims: The ubiquitously expressed Ca2+ Activated Cl- Channel Ano6 participates in the stimulation of cell membrane scrambling. Defective Ano6 underlies the Scott syndrome, an inherited bleeding disorder with impaired scrambling of plasma membrane phospholipids. At least in theory, the bleeding disorder of Scott syndrome may result from impaired platelet function. Activators of platelets include thrombin and collagen related peptide (CRP), which trigger increase of cytosolic Ca2+-activity ([Ca2+]i), production of reactive oxygen species (ROS), degranulation, integrin activation, as well as cell shrinkage and phospholipid scrambling of the cell membrane. The present study thus explored whether Ano6 modifies activation-induced alterations of cytosolic Ca2+-activity ([Ca2+]i), degranulation (P-selectin exposure), integrin activation, phosphatidylserine exposure on the platelet surface and platelet volume. Methods: Platelets from mice lacking Ano6 (ano6-/-) were compared to platelets from corresponding wild-type mice (ano6+/+). [Ca2+]i was estimated from Fluo-3 fluorescence, ROS from DCFDA fluorescence, degranulation from P-selectin abundance, integrin activation from αIIbβ3-integrin abundance, phosphatidylserine abundance from annexin-V-binding, and cell volume from forward scatter. Results: Platelet number in blood was slightly higher in ano6-/- mice than in ano6+/+ mice. Without activation [Ca2+]i and volume were similar in ano6-/- and ano6+/+ platelets as well as ROS abundance, P-selectin abundance, αIIbβ3 integrin activation, and phosphatidylserine exposure were negligible in both genotypes. Thrombin (0.01 U/ml) and CRP (2 or 5 µg/ml) increased [Ca2+]i, ROS abundance, platelet degranulation, αIIbβ3 integrin activation, and triggered annexin-V-binding as well as cell shrinkage, all effects less pronounced in ano6-/- than in ano6+/+ platelets. Conclusions: Genetic knockout of Ano6 blunts thrombin- and CRP-induced activation and apoptosis of blood platelets.

Background: Blood platelets accomplish primary hemostasis following vascular injury and contribute to the orchestration of occlusive vascular disease. Platelets are activated by an increase of cytosolic Ca2+-activity ([Ca2+]i), which is accomplished by Ca2+-release from intracellular stores and subsequent store operated Ca2+ entry (SOCE) through Ca2+ release activated Ca2+ channel moiety Orai1. Powerful activators of platelets include thrombin and collagen related peptide (CRP), which are in part effective by activation of small G- protein Rac1. The present study explored the influence of thrombin and CRP on Orai1 protein abundance and cytosolic Ca2+-activity ([Ca2+]i) in platelets drawn from wild type mice. Methods: Orai1 protein surface abundance was quantified utilizing CF™488A conjugated antibodies, and [Ca2+]i was determined with Fluo3-fluorescence. Results: In resting platelets, Orai1 protein abundance and [Ca2+]i were low. Thrombin (0.02 U/ml) and CRP (5ug/ml) within 2 min increased [Ca2+]i and Orai1 protein abundance at the platelet surface. [Ca2+]i was further increased by Ca2+ ionophore ionomycin (1 µM) and by store depletion with the sarcoendoplasmatic Ca2+ ATPase inhibitor thapsigargin (1 µM). However, Orai1 protein abundance at the platelet surface was not significantly affected by ionomycin and only slightly increased by thapsigargin. The effect of thrombin and CRP on Orai1 abundance and [Ca2+]i was significantly blunted by Rac1 inhibitor NSC23766 (50 µM). Conclusion: The increase of [Ca2+]i following stimulation of platelets with thrombin and collagen related peptide is potentiated by ultrarapid Rac1 sensitive translocation of Orai1 into the cell membrane.

Background: The antimalarial drug mefloquine has previously been shown to stimulate apoptosis of nucleated cells. Similar to apoptosis, erythrocytes may enter suicidal death or eryptosis, which is characterized by cell shrinkage and phospholipid scrambling of the erythrocyte cell membrane with phosphatidylserine translocation to the erythrocyte surface. Stimulators of eryptosis include oxidative stress, increase of cytosolic Ca2+-activity ([Ca2+]i), and ceramide. Methods: Phosphatidylserine abundance at the cell surface was estimated from annexin V binding, cell volume from forward scatter, reactive oxidant species (ROS) from 2′,7′-dichlorodihydrofluorescein diacetate (DCFDA) fluorescence, [Ca2+]i from Fluo3-fluorescence, and ceramide abundance from specific antibody binding. Results: A 48 h treatment of human erythrocytes with mefloquine significantly increased the percentage of annexin-V-binding cells (≥5 µg/ml), significantly decreased forward scatter (≥5 µg/ml), significantly increased ROS abundance (5 µg/ml), significantly increased [Ca2+]i (7.5 µg/ml) and significantly increased ceramide abundance (10 µg/ml). The up-regulation of annexin-V-binding following mefloquine treatment was significantly blunted but not abolished by removal of extracellular Ca2+. Even in the absence of extracellular Ca2+, mefloquine significantly increased annexin-V-binding. Conclusions: Mefloquine treatment leads to erythrocyte shrinkage and erythrocyte membrane scrambling, effects at least partially due to induction of oxidative stress, increase of [Ca2+]i and up-regulation of ceramide abundance.