The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib–IX, mediates platelet adhesion and activation. The cytoplasmic domains of the GPIb α and β subunits contain binding sites for the phosphorylation-dependent signaling molecule, 14-3-3. Here we show that a novel membrane-permeable inhibitor of 14-3-3-GPIbα interaction, MPα C, potently inhibited VWF binding to platelets and VWF-mediated platelet adhesion under flow conditions. MPαC also inhibited VWF-dependent platelet agglutination induced by ristocetin. Furthermore, activation of the VWF binding function of GPIb-IX induced by GPIbβ dephosphorylation is diminished by mutagenic disruption of the 14-3-3 binding site in the C-terminal domain of GPIbα, mimicking MPα C-induced inhibition, indicating that the inhibitory effect of MPαC is likely to be caused by disruption of 14-3-3 binding to GPIbα. These data suggest a novel 14-3-3-dependent regulatory mechanism that controls the VWF binding function of GPIb-IX, and also suggest a new type of antiplatelet agent that may be potentially useful in preventing or treating thrombosis.

Calmodulin (CaM) antagonists induce apoptosis in various tumor models and inhibit tumor cell invasion and metastasis, thus some ofwhich have been extensively used as anti-cancer agents. In platelets, CaMhas been found to bind directly to the cytoplasmic domains of several platelet receptors. Incubation of platelets with CaM antagonists impairs the receptors-related platelet functions. However, it is still unknown whether CaM antagonists induce platelet apoptosis. Here we show that CaM antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7), tamoxifen (TMX), and trifluoperazine (TFP) induce apoptotic events in human platelets, including depolarization of mitochondrial inner transmembrane potential, caspase-3 activation, and phosphatidylserine exposure. CaMantagonists did not incur platelet activation as detected by P-selectin surface expression and PAC-1 binding. However, ADP-, botrocetin-, and α-thrombin-induced platelet aggregation, platelet adhesion and spreading on von Willebrand factor surface were significantly reduced in platelets pretreatedwith CaM antagonists. Furthermore, cytosolic Ca2+ levelswere obviously elevated by both W7and TMX, and membrane-permeable Ca2+ chelator BAPTA AMsignificantly reduced apoptotic events in platelets induced by W7. Therefore, these findings indicate that CaM antagonists induce platelet apoptosis. The elevation of the cytosolic Ca2+ levels may be involved in the regulation of CaM antagonists-induced platelet apoptosis.

Many serious thrombotic and haemorrhagic diseases or fatalities have been documented in human being exposed to microgravity or hypergravity environments, such as crewmen in space, roller coaster riders, and aircrew subjected to high-G training. Some possible related organs have been examined to explore the mechanisms underlying these gravity change-related diseases. However, the role of platelets which are the primary players in both thrombosis and haemostasis is unknown. Here we show that platelet aggregation induced by ristocetin or collagen and platelet adhesion to von Willebrand factor (VWF) were significantly decreased after platelets were exposed to simulated microgravity. Conversely, these platelet functions were increased after platelets were exposed to hypergravity. The tail bleeding time in vivo was significantly shortened in mice exposed to high-G force, whereas, was prolonged in hindlimb un-loaded mice. Furthermore, three of 23 mice died after 15 minutes of -8Gx stress. Platelet thrombi disseminated in the heart ventricle and blood vessels in the brain, lung, and heart from the dead mice. Finally, glycoprotein (GP) Ibα surface expression and its association with the cytoskeleton were significantly decreased in platelets exposed to simulated microgravity, and obviously increased in hypergravity-exposed platelets. These data indicate that the platelet functions are inhibited in microgravity environments, and activated under high-G conditions, suggesting a novel mechanism for gravity change-related haemorrhagic and thrombotic diseases. This mechanism has important implications for preventing and treating gravity change-related diseases, and also suggests that special attentions should be paid to human actions under different gravity conditions.

Thrombotic diseases or fatalities have been reported to occasionally occur under conditions of hypergravity although the mechanism is still unclear. To investigate the effect of hypergravity on platelets that are the primary players in thrombus formation, platelet rich plasma (PRP) or washed platelets were exposed to hypergravity at 8 G for 15 minutes. No platelet aggregation was induced by 8 G alone, whereas ristocetin or collagen-induced platelet aggregation was significantly increased. The number of platelets adherent to immobilized fibrinogen and the area of platelets spreading on von Willbrand factor (VWF) matrix were increased simultaneously. Flow cytometry assay indicated that integrin αIIbβ3 was partially activated in 8 Gexposed platelets, but there was no significant difference in P-selectin surface expression between platelets treated with 8G and 1G control. The results indicate that hypergravity leads to human platelet hyperactivity, but fails to incur essential platelet activation events, suggesting a novel mechanism for thrombotic diseases occurring under hypergravitional conditions.

Hemorrhage is a significant pathological feature of some fever or hyperthermia-related diseases, such as dengue fever and heatstroke. Although the mechanisms of hemorrhage in these diseases are thought to be complex, whether there is an association between hemorrhage and hyperthermia or fever remains unclear. Platelets play a central role in maintaining integrity of endothelium and biological hemostasis. To explore the effect of hyperthermia on platelet physiology, platelet-rich plasma or washed platelets were incubated at hypothermia (22℃), normothermia (37℃) or hyperthermia (40 and 42℃) for 1 or 2 hours. ADP and α-thrombin induced platelet aggregations were obviously reduced in platelets incubated at hyperthermia. Hyperthermia induced apoptotic events in platelets, including depolarization of mitochondrial inner transmembrane potential, caspase-3 dependent gelsolin cleavage and phosphatidylserine exposure. Furthermore, hyperthermia incurred platelet glycoprotein Ibβ ectodomain shedding. Thus, these data suggest that hyperthermia induces platelet apoptosis and dysfunction. These findings have important implications for the pathogenesis of hemorrhage in fever or hyperthermia-related diseases, and also suggest that attention should be paid to platelet apoptosis under relatively high temperature conditions.