Mer Receptor Tyrosine Kinase Signaling Participates in Platelet Function

Abstract

Objective-Recently, mice made deficient in growth arrest-specific gene 6 product (Gas6) or in which Gas6 gene expression was inhibited were shown to have platelet dysfunction and to be less susceptible to thrombosis. The aim of this study was to define and characterize the relevant Gas6 receptor or receptors involved in platelet function. Methods and Results-Using RT-PCR and Western blot analysis we found that mer was the predominantly expressed subtype in mouse and human platelets, whereas axl and rse were not detected. We generated mer-deficient mice by targeted disruption of the mer receptor gene. Platelets derived from mer-deficient mice had decreased platelet aggregation in responses to low concentrations of collagen, U46619, and PAR4 thrombin receptor agonist peptide in vitro. However, the response to ADP was not different from wild-type platelets. Knockout of the mer gene protected mice from collagen/epinephrine-induced pulmonary thromoembolism and inhibited ferric chloride-induced thrombosis in vivo. Tail bleeding times, coagulation parameters, and peripheral blood cell counts in mer-deficient mice were similar to wild-type mice. Key Words: receptor tyrosine kinase Ⅲ Gas6 Ⅲ mer Ⅲ platelet Ⅲ thrombosis M er (c-mer, Nyk, Eyk) 1,2 tyrosine kinase belongs to a family of transmembrane receptors that also includes axl (Ark, Ufo, Tyro7) 3 and rse (Tyro3, Dtk, Etk, Brt, Tif). 4,5 This family of proteins has been implicated in reversible cell growth arrest, 6 survival, 7 proliferation, 7-9 and cell adhesion. 10 -12 Recently, the product of the growth arrest-specific gene 6 (Gas6) was identified as a heterophilic ligand for axl, rse, and mer, whereas protein S was identified as the ligand for the rse receptor. [13][14][15][16] Both Gas6 and protein S belong to the vitamin K-dependent protein family. Proteins in this family, which also includes prothrombin; coagulation factors VII, IX, and X; protein C; and protein Z, are characterized by posttranslational ␥-carboxylation of certain glutamic acid residues by carboxylase, using vitamin K as a cofactor. 6,17 Gas6 is structurally similar to protein S with 48% amino acid identity. Protein S is a cofactor for activated protein C that inactivates the coagulation factors Va and VIIIa 18 and is thus involved in the anticoagulation cascade. Genetic deficiency of protein S in humans is one of the most severe inherited risk factors for thrombosis. 19 Gas6, originally isolated as a growth arrest-specific gene from quiescent fibroblasts, 6 has recently been shown to participate in the regulation of platelet activation and aggregation. 20 Gas6 amplified platelet aggregation and secretion in response to known platelet agonists including ADP, collagen, and the thromboxane A2 analogue U46619. 20 Inactivation of the Gas6 gene prevented venous and arterial thrombosis in mice, and protected against fatal collagen/epinephrine-induced thromboembolism. 20 In addition, Gas6 antibodies inhibited platelet aggregation in vitro and protected mice against fatal thromboembolism in vivo. 20 Con...

TAM receptors, Gas6, and protein S: roles in inflammation and hemostasis

“…34,35 Besides, only expression of the Mer receptor on human platelets has been shown. 54 Still, Gas6 levels in plasma were higher in patients with venous thromboembolic disease as compared with healthy volunteers. 55 Also, genetic evidence shows an association between certain single nucleotide polymorphisms in the GAS6 gene and stroke, 56 making involvement of TAM receptor in human, and not only murine, hemostasis likely.…”

JAM-A protects from thrombosis by suppressing integrin αIIbβ3-dependent outside-in signaling in platelets

“…29,30 The time for complete occlusion of the carotid artery after injury with 7.5% FeCl 3 was compared in age-and sex-matched Jam-A wt/wt and Jam-A gt/gt mice. We found that ablation of Jam-A significantly shortens the time required for complete and stable vessel occlusion on vessel injury ( Figure 1C-D, P ϭ .002), again suggesting a prothrombotic phenotype in Jam-A-deficient mice.…”

Tyro3 Family-Mediated Cell Entry of Ebola and Marburg Viruses

“…Gas6 binding to Axl or other members of the Tyro3 family induce cell type-specific effects, such as deactivation of stimulated macrophages (34). In the vasculature, Gas6-Tyro3 binding prevents endothelial cells from TNF-␣-induced cytotoxicity (37,41) and stabilizes platelet aggregation (2,11). Thus, by interfering with this binding, filovirus GPs could alter the physical functions of the Tyro3 family, resulting in dysregulation of the immune and vascular systems, a hallmark of filovirus infection (15).…”