Platelet aggregation, secretion and thrombus formation play a critical role in primary hemostasis to prevent excessive blood loss. On the other hand, uncontrolled platelet activation leads to pathological thrombus formation resulting in myocardial infarction or stroke. Stimulation of heterotrimeric G-proteins by soluble agonists or immunoreceptor tyrosine based activation motif-coupled receptors that interact with immobilized ligands such as the collagen receptor glycoprotein (GP) VI lead to the activation of phospholipases that cleave membrane phospholipids to generate soluble second messengers. Platelets contain the phospholipases (PL) D1 and D2 which catalyze the hydrolysis of phosphatidylcholine to generate the second messenger phosphatidic acid (PA). The production of PA is abrogated by primary alcohols that have been widely used for the analysis of PLD-mediated processes. However, it is not clear if primary alcohols effectively reduce PA generation or if they induce PLD-independent cellular effects. In the present study we made use of the specific PLD inhibitor 5-fluoro-2-indolyl des-chlorohalopemide (FIPI) and show for the first time, that FIPI enhances platelet dense granule secretion and aggregation of human platelets. Further, FIPI has no effect on cytosolic Ca(2+) activity but needs proper Rho kinase signaling to mediate FIPI-induced effects on platelet activation. Upon FIPI treatment the phosphorylation of the PKC substrate pleckstrin was prominently enhanced suggesting that FIPI affects PKC-mediated secretion and aggregation in platelets. Similar effects of FIPI were observed in platelets from mouse wild-type and Pld1(-/-) mice pointing to a new role for PLD2 as a negative regulator of platelet sensitivity.

Platelet activation and thrombus formation play a critical role in primary hemostasis but also represent a pathophysiological mechanism leading to acute thrombotic vascular occlusions. Besides, platelets modulate cellular processes including inflammation, angiogenesis and neurodegeneration. On the other hand, platelet activation and thrombus formation are altered in different diseases leading to either bleeding complications or pathological thrombus formation. For many years platelets have been considered to play a role in neuroinflammatory diseases such as Alzheimer's disease (AD). AD is characterized by deposits of amyloid-β (Aβ) and strongly related to vascular diseases with platelets playing a critical role in the progression of AD because exposure of platelets to Aβ induces platelet activation, platelet Aβ release, and enhanced platelet adhesion to collagen in vitro and at the injured carotid artery in vivo. However, the molecular mechanisms and the relation between vascular pathology and amyloid-β plaque formation in the pathogenesis of AD are not fully understood. Compelling evidence is suggestive for altered platelet activity in AD patients. Thus we analyzed platelet activation and thrombus formation in aged AD transgenic mice (APP23) known to develop amyloid-β deposits in the brain parenchyma and cerebral vessels. As a result, platelets are in a pre-activated state in blood of APP23 mice and showed strongly enhanced integrin activation, degranulation and spreading kinetics on fibrinogen surfaces upon stimulation. This enhanced platelet signaling translated into almost unlimited thrombus formation on collagen under flow conditions in vitro and accelerated vessel occlusion in vivo suggesting that these mice are at high risk of arterial thrombosis leading to cerebrovascular and unexpectedly to cardiovascular complications that might be also relevant in AD patients.