Fibrinogen controls human platelet fibronectin internalization and cell-surface retention

Background: We recently demonstrated that platelet aggregation occurred in fibrinogen-deficient mice. In these animals, platelet fibronectin (Fn) content was increased 35 fold, suggesting that Fn may also be involved in platelet aggregation. Methods and results: We compared platelet Fn content from a severe hypofibrinogenemic patient (with approximately 0.5% of normal fibrinogen levels) with his parents (heterozygous) and healthy donors. A significant increase in the patient's platelet Fn content was detected by immunoblot, flow cytometry, and immunoelectron microscopy (IEM). To examine the possible contribution of platelet Fn to platelet aggregation, we examined cell-surface Fit expression after thrombin treatment. Unexpectedly, IEM detected only trace amounts of Fn retained on the patient's platelet surface, and flow cytometry indicated that surface Fn was approximately 6-fold lower than that of his parents and tenfold lower than that of healthy donors. An ELISA further confirmed that the patient's platelet Fn was primarily released into the extracellular medium. To test whether retention of surface Fn was due to fibrin formation on the platelet surface, an antifibrin antibody (T2 G1) was employed. Fibrin was detected on platelets from healthy donors and from the father, but was negligible on the patient's platelets. Consistent with these data, when gel-filtered platelets of healthy donors were treated with thrombin receptor activation peptide (SFLLRN-NH2; no conversion of fibrinogen to fibrin), little surface Fn was detected. Conclusion: Fibrinogen not only competitively inhibits human platelet Fn internalization but also controls platelet-surface Fn retention via fibrin formation. The Fn-fibrin interaction is one possible mechanism to promote Fn interaction with platelets.