1. MLAB 1227- Coagulation Keri Brophy-Martinez Fibrinolytic System

2. Fibrinolysis Process of removing fibrin from the vasculature Key Players ◦ Plasminogen (PLG) ◦ Plasminogen activators (PA) ◦ Active enzyme plasmin (PLN) ◦ Fibrin ◦ Fibrin/Fibrinogen degradation products ◦ Plasminogen activator inhibitor

3. Fibrinolytic System Sensitive to imbalances Restricts fibrin formation to area of injury Initiated when coagulation cascade begins Dissolves clot by digestion of fibrin

4. Overview Under the influence of thrombin. Fibrinogen cleaved into fibrin monomers Fibrin monomers are cleaved into fibrin degradation products or fibrin split products

5. Process summary Once clotting begins, the fibrinolytic system comes to life 1.Plasminogen (PLG) binds to fibrin in the developing thrombus 2.Tissue-type PLG activator (tPA) also binds to fibrin, increasing its enzymatic activity to convert plasminogen to plasmin (PLN) 3.Complex formation of tPA, PLG and fibrin results in the break-down of fibrin 4.PLN then further digest fibrin to soluble degradation products making fibrin fragments

6. Plasminogen ◦ Produced in the liver ◦ Found in normal plasma ◦ Following injury, binds to fibrin during clot formation along with plasminogen activators

7. Activators of Fibrinolysis Contact Phase/Intrinsic ◦ Occurs by interactions of the contact factors (XIIa, HMWK, and PK) following intrinsic pathway activation(collagen exposure) Physiologic  Activators released from tissues extrinsic to the blood  tPA: tissue- type PLG activator  Found in endothelial cells of small vessels  uPA: urokinase-type PLG activator  Made in renal tubular epithelium and vascular epithelium  Found in urine and plasma ◦ Exogenous activation  Via medications given to lyse pathogenic clots (i.e. pulmonary emboli)  Example includes Streptokinase

8. Activators

9. Plasmin Proteolytic enzyme Dissolves fibrin/fibrinogen clots into protein fragments that are cleared from plasma by the liver Provides a positive feedback loop for forming more plasminogen Highly regulated Temporarily active Local

10. Fibrinogen and Fibrin Fibrin degradation products are the protein fragments of fibrin or fibrinogen. ◦ The protein fragments are designated X, Y, D, and E ◦ Fragments are strong inhibitors of further coagulation by  interfering with the action of thrombin  interfering with platelet aggregation  In the lab, referred to as FDPs or FSPs  Fibrin degradation products are cleared by the liver Fibrinogen and fibrin yield essentially the same fragments; however degradation of cross-linked fibrin is slower and leads to fragments that contain D-dimer.

11. Plasmin Degradation of Fibrinogen D D D D D D E D E E E plasmin Fibrinogen Fragment X: small peptides from carboxyl end of α chain removed Fragment Y + Fragment D Fragment D + Fragment E

12. Plasminogen Intrinsic/contact activation Physiologic activation Exogenous activation Plasmin Fibrin clot Fibrinogen Fibrin Degradation Fibrinogen Products Degradation Products X,Y,D=D,E X,Y,D,E,D

13. Inhibitors of Fibrinolysis Used to regulate and limit plasmin activity and fibrinolysis Also referred to as an antiplasmin How? ◦ Target plasminogen activation step ◦ Target plasmin

14. Inhibitors of Fibrinolysis Inhibitors

15. Specific Inhibitors Plasminogen Activator Inhibitor (PAI) ◦ PAI-1: most significant ◦ Inhibits tPA and uPA ◦ Acute phase reactant protein Thrombin Activatable Fibrinolysis Inhibitor (TAFI) ◦ Eliminates fibrin binding sites for plasminogen

16. Specific Inhibitors alpha-2-antiplasmin ◦ Rapid inhibitor of plasmin ◦ Functions to “catch” leaked plasmin in the circulation, thus limiting activity to fibrin clot ◦ Produced in liver and α granules in platelets alpha-2-macroglobulin ◦ Slower inhibitor of plasmin

17. References McKenzie, Shirlyn B., and J. Lynne. Williams. "Chapter 30." Clinical Laboratory Hematology. Boston: Pearson, 2010. Print.