title: Tissue-factor-bearing microvesicles arise from lipid rafts and fuse with activated platelets to initiate coagulation.
abstract: Tissue factor (TF) circulates in plasma, largely on monocyte/macrophage-derived microvesicles that can bind activated platelets through a mechanism involving P-selectin glycoprotein ligand-1 (PSGL-1) on the microvesicles and P-selectin on the platelets. We found these microvesicles to be selectively enriched in both TF and PSGL-1, and deficient in CD45, suggesting that they arise from distinct membrane microdomains. We investigated the possibility that microvesicles arise from cholesterol-rich lipid rafts and found that both TF and PSGL-1, but not CD45, localize to lipid rafts in blood monocytes and in the monocytic cell line THP-1. Consistent with a raft origin of TF-bearing microvesicles, their shedding was significantly reduced with depletion of membrane cholesterol. We also evaluated the interaction between TF-bearing microvesicles and platelets. Microvesicles bound only activated platelets, and required PSGL-1 to do so. The microvesicles not only bound the activated platelets, they fused with them, transferring both proteins and lipid to the platelet membrane. Fusion was blocked by either annexin V or an antibody to PSGL-1 and had an important functional consequence: increasing the proteolytic activity of the TF-VIIa complex. These findings suggest a mechanism by which all of the membrane-bound reactions of the coagulation system can be localized to the surface of activated platelets.
authors.affiliation: Thrombosis Research Section, Department of Medicine, Baylor College of Medicine, One Baylor Plaza, N-1317, Houston, TX 77030, USA.
authors.forename: Ian
authors.initials: I
authors.lastname: Del Conde
authors.forename: Corie N
authors.initials: CN
authors.lastname: Shrimpton
authors.forename: Perumal
authors.initials: P
authors.lastname: Thiagarajan
authors.forename: José A
authors.initials: JA
authors.lastname: López
mesh_terms: D001777:Blood Coagulation / Q000502:physiology*; D001792:Blood Platelets / Q000378:metabolism*; D002784:Cholesterol / Q000097:blood / Q000494:pharmacology; D022162:Cytoplasmic Vesicles / Q000187:drug effects / Q000378:metabolism*; D015942:Factor VIIa / Q000378:metabolism; D006801:Humans; D017493:Leukocyte Common Antigens / Q000378:metabolism; D008561:Membrane Fusion / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000378:metabolism; D021962:Membrane Microdomains / Q000378:metabolism*; D015539:Platelet Activation / Q000502:physiology; D013925:Thromboplastin / Q000378:metabolism / Q000502:physiology*; D013997:Time Factors
chemical_list: D008562:Membrane Glycoproteins; C084882:P-selectin ligand protein; D013925:Thromboplastin; D002784:Cholesterol; D017493:Leukocyte Common Antigens; D015942:Factor VIIa