Exposure of plasminogen and a novel plasminogen receptor, Plg-RKT, on activated human and murine platelets

Abstract

Plasminogen activation rates are enhanced by cell surface binding. We previously demonstrated that exogenous plasminogen binds to phosphatidylserine-exposing and spread platelets. Platelets contain plasminogen in their α-granules, but secretion of plasminogen from platelets has not been studied. Recently, a novel transmembrane lysine-dependent plasminogen receptor, Plg-RKT, has been described on macrophages. Here, we analyzed the pool of plasminogen in platelets and examined whether platelets express Plg-RKT. Plasminogen content of the supernatant of resting and collagen/thrombin-stimulated platelets was similar. Pretreatment with the lysine analog, ∈-aminocaproic acid, significantly increased platelet-derived plasminogen (0.33 vs 0.08 nmol/108platelets) in the stimulated supernatant, indicating a lysine-dependent mechanism of membrane retention. Lysine-dependent, platelet-derived plasminogen retention on thrombin and convulxin activated human platelets was confirmed by flow cytometry. Platelets initiated fibrinolytic activity in fluorescently labeled plasminogen-deficient clots and in turbidimetric clot lysis assays. A 17-kDa band, consistent with Plg-RKT, was detected in the platelet membrane fraction by western blotting. Confocal microscopy of stimulated platelets revealed Plg-RKTcolocalized with plateletderived plasminogen on the activated platelet membrane. Plasminogen exposure was significantly attenuated in thrombin- and convulxin-stimulated platelets from Plg-RKT-/-mice compared with Plg-RKT+/+littermates. Membrane exposure of Plg-RKTwas not dependent on plasminogen, as similar levels of the receptor were detected in plasminogen-/-platelets. These data highlight Plg-RKTas a novel plasminogen receptor in human and murine platelets. We show for the first time that platelet-derived plasminogen is retained on the activated platelet membrane and drives local fibrinolysis by enhancing cell surface-mediated plasminogen activation. (Blood. 2021;137(2):248-257).