• Christopher W Smith, Steven G Thomas, Zaher Raslan, Pushpa Patel, Maxwell Byrne, Marie Lordkipanidzé, Danai Bem, Linde Meyaard, Yotis A Senis, Steve P Watson, and Alexandra Mazharian.
• From the Institute of Cardiovascular Sciences, College of Medical and Dental Sciences (C.W.S., S.G.T., Z.R., P.P., M.B., M.L., Y.A.S., S.P.W., A.M.), and Institute of Applied Health Research, College of Medical and Dental Sciences (D.B.), University of Birmingham, United Kingdom; and Laboratory of Translational Immunology, Department of Immunology, University Medical Center Utrecht, the Netherlands (L.M.).
• Arterioscler. Thromb. Vasc. Biol. 2017 May 1; 37 (5): 823-835.

ObjectiveLeukocyte-associated immunoglobulin-like receptor-1 (LAIR-1) is a collagen receptor that belongs to the inhibitory immunoreceptor tyrosine-based inhibition motif-containing receptor family. It is an inhibitor of signaling via the immunoreceptor tyrosine-based activation motif-containing collagen receptor complex, glycoprotein VI-FcRγ-chain. It is expressed on hematopoietic cells, including immature megakaryocytes, but is not detectable on platelets. Although the inhibitory function of LAIR-1 has been described in leukocytes, its physiological role in megakaryocytes and in particular in platelet formation has not been explored. In this study, we investigate the role of LAIR-1 in megakaryocyte development and platelet production by generating LAIR-1-deficient mice.Approach And ResultsMice lacking LAIR-1 exhibit a significant increase in platelet counts, a prolonged platelet half-life in vivo, and increased proplatelet formation in vitro. Interestingly, platelets from LAIR-1-deficient mice exhibit an enhanced reactivity to collagen and the glycoprotein VI-specific agonist collagen-related peptide despite not expressing LAIR-1, and mice showed enhanced thrombus formation in the carotid artery after ferric chloride injury. Targeted deletion of LAIR-1 in mice results in an increase in signaling downstream of the glycoprotein VI-FcRγ-chain and integrin αIIbβ3 in megakaryocytes because of enhanced Src family kinase activity.ConclusionsFindings from this study demonstrate that ablation of LAIR-1 in megakaryocytes leads to increased Src family kinase activity and downstream signaling in response to collagen that is transmitted to platelets, rendering them hyper-reactive specifically to agonists that signal through Syk tyrosine kinases, but not to G-protein-coupled receptors.© 2017 American Heart Association, Inc.

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