Deficiency of Src homology 2 domain–containing inositol 5-phosphatase 1 affects platelet responses and thrombus growth

Abstract

Platelets are critical for normal hemostasis. Their deregulation can lead to bleeding or to arterial thrombosis, a primary cause of heart attack and ischemic stroke. Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is a 5-phosphatase capable of dephosphorylating the phosphatidylinositol 3,4,5-trisphosphate second messenger into phosphatidylinositol 3,4-bisphosphate. SHIP1 plays a critical role in regulating the level of these 2 lipids in platelets. Using SHIP1-deficient mice, we found that its loss affects platelet aggregation in response to several agonists with minor effects on fibrinogen binding and β 3 integrin tyrosine phosphorylation. Accordingly, SHIP1-null mice showed defects in arterial thrombus formation in response to a localized laser-induced injury. Moreover, these mice had a prolonged tail bleeding time. Upon stimulation, SHIP1-deficient platelets showed large membrane extensions, abnormalities in the open canalicular system, and a dramatic decrease in close cell-cell contacts. Interestingly, SHIP1 appeared to be required for platelet contractility, thrombus organization, and fibrin clot retraction. These data indicate that SHIP1 is an important element of the platelet signaling machinery to support normal hemostasis. To our knowledge, this is the first report unraveling an important function of SHIP1 in the activation of hematopoietic cells, in contrast to its well-documented role in the negative regulation of lymphocytes. IntroductionPlatelet activation is a highly regulated process and critical for mediating normal hemostasis. Vascular injury exposes subendothelial matrix proteins that allow platelet arrest and their subsequent spreading, activation, and secretion of soluble mediators. These soluble agonists recruit circulating platelets, allowing aggregation for the bleeding to stop. In pathological states, platelet activation can contribute to thrombosis and cerebrovascular and arterial occlusion and eventually to heart attacks and strokes. Several signaling mechanisms have been shown to contribute to platelet activation, including the phospholipase C (PLC)/Ca 2+ and the PI3K pathways. By generating phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P 3 ] and phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P 2 ] second messengers, PI3K plays an important role in platelet activation and plug stability (1-3). Using a selective PI3K β inhibitor, Jackson et al. (3) recently suggested that this lipid kinase could be a potential target for antithrombotic therapy.It is now well established in several models that phosphoinositide phosphatases involved in the hydrolysis of PtdIns(3,4,5)P 3 and PtdIns(3,4)P 2 , such as the phosphatase and tensin homolog (PTEN), are essential for the negative regulation of PI3K pathways (4). PTEN is commonly deleted or inactivated in several cancers,

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