AGK Potentiates Arterial Thrombosis by Affecting Talin-1 and αIIbß3-Mediated Bidirectional Signaling Pathway.

BACKGROUND: AGK (acylglycerol kinase) was first identified as a mitochondrial transmembrane protein that exhibits a lipid kinase function. Recent studies have established that AGK promotes cancer growth and metastasis, enhances glycolytic metabolism and function fitness of CD8+ T cells, or regulates megakaryocyte differentiation. However, the role of AGK in platelet activation and arterial thrombosis remains to be elaborated. METHODS: We performed hematologic analysis using automated hematology analyzer and investigated platelets morphology by transmission electron microscope. We explored the role of AGK in platelet activation and arterial thrombosis utilizing transgenic mice, platelet functional experiments in vitro, and thrombosis models in vivo. We revealed the regulation effect of AGK on Talin-1 by coimmunoprecipitation, mass spectrometry, immunofluorescence, and Western blot. We tested the role of AGK on lipid synthesis of phosphatidic acid/lysophosphatidic acid and thrombin generation by specific Elisa kits. RESULTS: In this study, we found that AGK depletion or AGK mutation had no effect on the platelet average volumes, the platelet microstructures, or the expression levels of the major platelet membrane receptors. However, AGK deficiency or AGK mutation conspicuously decreased multiple aspects of platelet activation, including agonists-induced platelet aggregation, granules secretion, JON/A binding, spreading on Fg (fibrinogen), and clot retraction. AGK deficiency or AGK mutation also obviously delayed arterial thrombus formation but had no effect on tail bleeding time and platelet procoagulant function. Mechanistic investigation revealed that AGK may promote Talin-1Ser425 phosphorylation and affect the αIIbß3-mediated bidirectional signaling pathway. However, AGK does not affect lipid synthesis of phosphatidic acid/lysophosphatidic acid in platelets. CONCLUSIONS: AGK, through its kinase activity, potentiates platelet activation and arterial thrombosis by promoting Talin-1 Ser425 phosphorylation and affecting the αIIbß3-mediated bidirectional signaling pathway.

Role of GPR56 in Platelet Activation and Arterial Thrombosis.

The adhesion G protein-coupled receptor GPR56 mediates cell-cell and cell-extracellular matrix interactions. To examine the function of GPR56 in platelet activation and arterial thrombosis, we generated GPR56-knockout mice and evaluated GPR56 expression in human and mouse platelets. The results revealed that the levels of the GPR56 N-terminal fragment were significantly higher on the first day after myocardial infarction than on the seventh day in the plasma of patients with ST-segment-elevation myocardial infarction. Next, we investigated the effects of GPR56 on platelet function in vitro and in vivo. We observed that collagen-induced aggregation and adenosine triphosphate release were reduced in Gpr56 -/- platelets. Furthermore, P-selectin expression on the Gpr56 -/- platelet surface was also reduced, and the spreading area on immobilized collagen was decreased in Gpr56 -/- platelets. Furthermore, collagen-induced platelet activation in human platelets was inhibited by an anti-GPR56 antibody. Gpr56 -/- mice showed an extended time to the first occlusion in models with cremaster arteriole laser injury and FeCl3-induced carotid artery injury. GPR56 activated the G protein 13 signaling pathway following collagen stimulation, which promoted platelet adhesion and thrombus formation at the site of vascular injury. Thus, our study confirmed that GPR56 regulated the formation of arterial thrombosis. Inhibition of the initial response of GPR56 to collagen could significantly inhibit platelet activation and thrombus formation. Our results provide new insights for research into antiplatelet drugs.

Comparison of multiple imaging modalities for measuring orifice diameter and selecting occluder size in patients undergoing left atrial appendage closure.

BACKGROUND: Left atrial appendage (LAA) closure (LAAC) can safely and effectively prevent stroke events caused by atrial fibrillation. However, the structure of the LAA is highly variable among individuals, and the optimal method for obtaining measurements remains unknown. HYPOTHESIS: We aimed to study the accuracy of left atrial computed tomography angiography (CTA), three-dimensional (3D) reconstruction using CTA, two-dimensional transesophageal echocardiography (2D-TEE), and digital subtraction angiography (DSA) for measuring the diameter of the LAA and compare their value for selecting occluder size. METHODS: We retrospectively evaluated data for 148 patients with nonvalvular atrial fibrillation who underwent successful LAAC. CTA and 2D-TEE of the left atrium and pulmonary vein were performed before LAAC. We performed 3D reconstruction of the left atrium and LAA using Mimics and 3-matics software. DSA of the LAA was performed during surgery. RESULTS: Values measured via CTA 3D reconstruction were significantly higher than those measured using other methods. DSA-measured values were significantly lower than those measured via CTA and CTA 3D reconstruction. Occluder size was positively correlated with LAA orifice diameter. The differences between occluder size and DSA, 2D-TEE, CTA, CTA 3D reconstruction measurements were 4.96 ± 2.58, 4.64 ± 2.50, 4.04 ± 1.37, and 2.92 ± 1.38 mm, respectively. Intraclass correlation coefficients for these methods were -.067, .006, .241, and .519, respectively. CONCLUSION: CTA 3D reconstruction provides the best correlation and consistency between the measured LAA orifice diameter and occluder size. Adding 2-4 mm to the maximum LAA orifice diameter based on 3D-CTA may aid in selecting the appropriate WATCHMAN device.

Paired immunoglobulin-like receptor B regulates platelet activation.

Murine paired immunoglobulin-like receptors B (PIRB), as the ortholog of human leukocyte immunoglobulin-like receptor B2 (LILRB2), is involved in a variety of biological functions. Here, we found that PIRB and LILRB2 were expressed in mouse and human platelets, respectively. PIRB intracellular domain deletion (PIRB-TM) mice had thrombocythemia and significantly higher proportions of megakaryocytes in bone marrow. Agonist-induced aggregation and spreading on immobilized fibrinogen were facilitated in PIRB-TM platelets. The rate of clot retraction in platelet-rich plasma containing PIRB-TM platelets was also increased. Characterization of signaling confirmed that PIRB associated with phosphatases Shp1/2 in platelets. The phosphorylation of Shp1/2 was significantly downregulated in PIRB-TM platelets stimulated with collagen-related peptide (CRP) or on spreading. The results further revealed that the phosphorylation levels of the linker for activation of T cells, SH2 domain-containing leukocyte protein of 76kDa, and phospholipase C were enhanced in PIRB-TM platelets stimulated with CRP. The phosphorylation levels of FAK Y397 and integrin ß3 Y759 were also enhanced in PIRB-TM platelet spread on fibrinogen. The PIRB/LILRB2 ligand angiopoietin-like-protein 2 (ANGPTL2) was expressed and stored in platelet α-granules. ANGPTL2 inhibited agonist-induced platelet aggregation and spreading on fibrinogen. The data presented here reveal that PIRB and its ligand ANGPTL2 possess an antithrombotic function by suppressing collagen receptor glycoprotein VI and integrin αIIbß3-mediated signaling.