title: The Gas6-Axl Protein Interaction Mediates Endothelial Uptake of Platelet Microparticles.
abstract: Upon activation, platelets release plasma membrane-derived microparticles (PMPs) exposing phosphatidylserine on their surface. The functions and clearance mechanism of these microparticles are incompletely understood. As they are pro-coagulant and potentially pro-inflammatory, rapid clearance from the circulation is essential for prevention of thrombotic diseases. The tyrosine kinase receptors Tyro3, Axl, and Mer (TAMs) and their ligands protein S and Gas6 are involved in the uptake of phosphatidylserine-exposing apoptotic cells in macrophages and dendritic cells. Both TAMs and their ligands are expressed in the vasculature, the functional significance of which is poorly understood. In this study, we investigated how vascular TAMs and their ligands may mediate endothelial uptake of PMPs. PMPs, generated from purified human platelets, were isolated by ultracentrifugation and labeled with biotin or PKH67. The uptake of labeled microparticles in the presence of protein S and Gas6 in human aortic endothelial cells and human umbilical vein endothelial cells was monitored by flow cytometry, Western blotting, and confocal/electron microscopy. We found that both endothelial cell types can phagocytose PMPs, and by using TAM-blocking antibodies or siRNA knockdown of individual TAMs, we show that the uptake is mediated by endothelial Axl and Gas6. As circulating PMP levels were not altered in Gas6(-/-) mice compared with Gas6(+/+) mice, we hypothesize that the Gas6-mediated uptake is not a means to clear the bulk of circulating PMPs but may serve to locally phagocytose PMPs generated at sites of platelet activation and as a way to effect endothelial responses.
authors.affiliation: From the Department of Translational Medicine, Division of Clinical Chemistry, Lund University, SE-20502 Malmö Sweden, kaisa.happonen@med.lu.se.
authors.forename: Kaisa E
authors.initials: KE
authors.lastname: Happonen
authors.affiliation: From the Department of Translational Medicine, Division of Clinical Chemistry, Lund University, SE-20502 Malmö Sweden.
authors.forename: Sinh
authors.initials: S
authors.lastname: Tran
authors.affiliation: the Department of Clinical Sciences, Division of Infection Medicine, Lund University, SE-22185 Lund, Sweden.
authors.forename: Matthias
authors.initials: M
authors.lastname: Mörgelin
authors.affiliation: the University Clinic of Hematology and Central Hematology Laboratory, Bern University Hospital, Bern CH-3010, Switzerland, and the Department of Clinical Research, University of Bern, Bern CH-3010, Switzerland.
authors.forename: Raja
authors.initials: R
authors.lastname: Prince
authors.affiliation: the University Clinic of Hematology and Central Hematology Laboratory, Bern University Hospital, Bern CH-3010, Switzerland, and the Department of Clinical Research, University of Bern, Bern CH-3010, Switzerland.
authors.forename: Sara
authors.initials: S
authors.lastname: Calzavarini
authors.affiliation: the University Clinic of Hematology and Central Hematology Laboratory, Bern University Hospital, Bern CH-3010, Switzerland, and the Department of Clinical Research, University of Bern, Bern CH-3010, Switzerland.
authors.forename: Anne
authors.initials: A
authors.lastname: Angelillo-Scherrer
authors.affiliation: From the Department of Translational Medicine, Division of Clinical Chemistry, Lund University, SE-20502 Malmö Sweden.
authors.forename: Björn
authors.initials: B
authors.lastname: Dahlbäck
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D055252:Cell-Derived Microparticles / Q000235:genetics / Q000378:metabolism*; D061307:Human Umbilical Vein Endothelial Cells / Q000166:cytology / Q000378:metabolism*; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D000099255:Growth Arrest-Specific Protein 6
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D020794:Receptor Protein-Tyrosine Kinases; D000099255:Growth Arrest-Specific Protein 6; C084944:TYRO3 protein, human
keywords: Axl; Gas6; endothelium; extracellular vesicles; microparticles; phagocytosis; platelet