The growth arrest-specific gene 6 product (Gas6) is a secreted protein related to the anticoagulant protein S but its role in hemostasis is unknown. Here we show that inactivation of the Gas6 gene prevented venous and arterial thrombosis in mice, and protected against fatal collagen/epinephrine-induced thrombo embolism. Gas6-/- mice did not, however, suffer spontaneous bleeding and had normal bleeding after tail clipping. In addition, we found that Gas6 antibodies inhibited platelet aggregation in vitro and protected mice against fatal thrombo embolism without causing bleeding in vivo. Gas6 amplified platelet aggregation and secretion in response to known agonists. Platelet dysfunction in Gas6-/- mice resembled that of patients with platelet signaling transduction defects. Thus, Gas6 is a platelet-response amplifier that plays a significant role in thrombosis. These findings warrant further evaluation of the possible therapeutic use of Gas6 inhibition for prevention of thrombosis.

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The role of Gas6 in endothelial cell (EC) function remains incompletely characterized. Here we report that Gas6 amplifies EC activation in response to inflammatory stimuli in vitro. In vivo, Gas6 promotes and accelerates the sequestration of circulating platelets and leukocytes on activated endothelium as well as the formation and endothelial sequestration of circulating platelet-leukocyte conjugates. In addition, Gas6 promotes leukocyte extravasation, inflammation, and thrombosis in mouse models of inflammation (endotoxinemia, vasculitis, heart trans- IntroductionThe growth arrest-specific gene 6 (Gas6) binds to the receptor tyrosine kinases Axl, Tyro3, and Mer. 1 Gas6 is composed of a N-terminal gamma-carboxy-glutamic acid domain (Gla-domain), a loop region, 4 EGF-like repeats, and a C-terminal steroid hormone binding globulin-like (SHBG-like) domain). 1 Even though this molecule was discovered as a homologue of the anticoagulant protein S more than a decade ago, its role in vivo remains incompletely characterized. 2,3 Originally identified in fibroblasts, Gas6 is expressed in various cell types, including endothelial cells (ECs), 4 smooth muscle, 5 and bone marrow cells. 6 Gas6 and its receptors modify platelet activation and aggregation, 7-10 but the role of Gas6 in the interplay between platelets and other cell types, such as ECs and leukocytes, 11 during inflammatory conditions remains unclear.Several lines of evidence indeed suggest that Gas6 may affect ECs and leukocytes. ECs and leukocytes express Gas6 and its receptors, especially in conditions of inflammation and repair. 4,[12][13][14][15][16] Gas6 stimulates EC survival [17][18][19][20] and promotes angiogenesis by enforcing the adhesion of Axl-expressing ECs via homophilic interactions, 21-23 yet another study suggested that activation of Axl impairs tyrosine phosphorylation of vascular endothelial growth factor (VEGF) receptor-2. 24 The activity of Gas6 on leukocytes also remains incompletely understood. Indeed, genetic loss of Mer inhibits cytokine production by natural killer cells 25 while it stimulates tumor necrosis factor-␣ (TNF-␣) production by monocytes 14 and impairs clearance of apoptotic cells. 26 Loss of all 3 Gas6 receptors, on the other hand, induces lymphoproliferative disorders via hyperactivation of antigen-presenting cells, 27,28 but mice lacking Gas6 (Gas6 Ϫ/Ϫ ) do not develop autoimmune health problems (P.C., unpublished data, 2008). In humans, the plasma levels of Gas6 were found to be elevated during severe sepsis, a life-threatening condition involving increased interactions between ECs, leukocytes, and platelets. 29,30 However, exogenous Gas6 inhibits granulocyte adhesion to ECs, but only at very high doses. 31 Furthermore, the role of endogenous Gas6 in leukocyte extravasation in vivo was not studied. Here, by using our previously generated Gas6 Ϫ/Ϫ mice, 7 we studied whether Gas6 might play a role in EC activation and in the interactions between ECs, platelets, and leukocytes during inflammatory conditions. Methods MiceTh...