Haemostasis 2 (2023-24).pdf

WEEK 26 MPharm Programme Haemostasis 2 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 The Coagulation Pathways Coagulation cascade traditionally divided into 3 interacting pathways – intrinsic, extrinsi...

WEEK 26 MPharm Programme Haemostasis 2 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 The Coagulation Pathways Coagulation cascade traditionally divided into 3 interacting pathways – intrinsic, extrinsic & common pathways Each pathway involves reactions with specific group of clotting factors Intrinsic – all the components occur in blood stream Extrinsic – requires a factor that is not normally present in the blood Both the intrinsic & extrinsic systems eventually converge on & activate the common pathway Slide 30 of 54 MPharm PHA112 Haemostasis WEEK 26 Common Pathway Slide 31 of 54 MPharm PHA112 Haemostasis WEEK 26 Components of the Coagulation Cascade Intrinsic Extrinsic Prekallikrein HMW Kininogens Factor XII Factor XI Factor IX Factor VIII Slow Tissue factor Factor VII Rapid Common Factor X Factor V Factor II (Prothrombin) Factor I (Fibrinogen) Fibrin Slide 32 of 54 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 Traditional Coagulation Pathways The Extrinsic Pathway Quick-responding – begins when damage occurs to the surrounding tissues, e.g. in a traumatic injury Involves interaction between tissue factor (as cofactor), FVII and Ca++ Tissue factor (tissue thromboplastin, Factor III) The principal or main initiator of coagulation in vivo Cell-bound glycoprotein – largely expressed in extravascular tissues, e.g. fibroblasts, smooth muscle cells Exposed to blood when the endothelial barrier is breached Slide 33 of 54 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 Traditional Coagulation Pathways The Extrinsic Pathway (cont’d) Upon contact with blood, damaged extravascular cells release tissue factor Released tissue factor (TF) forms a complex with FVII & Ca++ (called “extrinsic tenase” – extrinsic pathway Factor X activator) The TF-FVIIa complex, in turn, activates FX of the Common Pathway  thrombin (FIIa) generation The TF-FVIIa complex can also activate Factor IX of the Intrinsic Pathway (so-called “alternate pathway”) Slide 34 of 54 MPharm PHA112 Haemostasis WEEK 26 The Extrinsic Coagulation Pathway Activation of FX Slide 35 of 54 MPharm PHA112 Haemostasis WEEK 26 Activation of The Common Pathway by The Extrinsic Coagulation Pathway Slide 36 of 54 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 Traditional Coagulation Pathways The Intrinsic Pathway Slower-responding – begins when blood comes into contact with collagen in damaged blood vessel wall FXII (Hageman factor) forms a primary complex with high-molecular-weight kininogen (HMWK) & prekallikrein on collagen  activation of FXII FXIIa activates factor XI; FXIa, in turn, activates factor IX FIXa then forms a complex with FVIIIa (as cofactor), Ca++ & phosphatidylserine (“intrinsic tenase”) which, in turn, activates FX (Common Pathway)  thrombin generation Slide 37 of 54 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 Traditional Coagulation Pathways The Common Pathway Both intrinsic & extrinsic pathways activate the common pathway  production of fibrin to seal off the breach in blood vessel wall Consists of 5 different steps Activation of FX Conversion of prothrombin (FII) to thrombin (FIIa) Cleavage of fibrinogen (FI) to fibrin (FIa) Polymerisation of fibrin Stabilisation of fibrin polymers by FXIIIa Slide 38 of 54 MPharm PHA112 Haemostasis The Coagulation Cascade WEEK 26 Traditional Coagulation Pathways The Common Pathway (cont’d) FX is activated by the extrinsic & intrinsic tenase complexes of the Extrinsic & Intrinsic Pathways FXa then forms a complex with FVa, Ca++ and phosphatidylserine (“prothrombinase” complex) The “prothrombinase” complex then converts prothrombin (FII) to thrombin (FIIa) Thrombin (FIIa) cleaves fibrinogen to fibrin monomers Fibrin monomers undergo spontaneous polymerisation FXIIIa induces cross-linking & stabilisation of fibrin polymers Slide 39 of 54 MPharm PHA112 Haemostasis WEEK 26 Slide 40 of 54 MPharm PHA112 Haemostasis Secondary Haemostasis WEEK 26 Modern Cell-Based Model Limitations of Coagulation Cascade Model Doesn’t fully explain how blood clots in vivo Assumes that both the intrinsic & extrinsic pathways are independently capable of initiating clot formation Deficiency in the initial components of the intrinsic pathway – FXII, HMWK, or prekallikrein – does not lead to bleeding tendency Factor VIII or IX deficiency leads to a serious bleeding tendency, although the extrinsic pathway is intact FVII deficiency leads to a serious bleeding tendency, although the intrinsic pathway is intact Slide 41 of 54 MPharm PHA112 Haemostasis Secondary Haemostasis WEEK 26 Modern Cell-Based Model New Proposed Model Secondary haemostasis occurs in distinct, but overlapping, steps: Initiation Amplification Propagation The process requires the participation of 2 cell types: TF-bearing cells Platelets 2 cell types are kept apart until vascular injury occurs Slide 42 of 54 MPharm PHA112 Haemostasis WEEK 26 Secondary Haemostasis – Cell-Based Model Slide 43 of 54 MPharm PHA112 Haemostasis WEEK 26 Secondary Haemostasis – Cell-Based Model Slide 44 of 54 MPharm PHA112 Haemostasis WEEK 26 Overview of Primary & Secondary Haemostasis Slide 45 of 54 MPharm PHA112 Haemostasis WEEK Tertiary Haemostasis 26 Fibrinolysis Process that dissolves & removes the fibrin clot following secondary haemostasis Ensures localisation of fibrin clot formation & clot removal after wound healing Mainly mediated by plasmin, a proteolytic enzyme that degrades the fibrin mesh Process involves Release of plasminogen activators Plasmin production from inactive precursor, plasminogen Clot lysis & release of degradation products Slide 46 of 54 MPharm PHA112 Haemostasis WEEK Fibrinolysis 26 Slide 47 of 54 MPharm PHA112 Haemostasis WEEK Fibrinolysis 26 Slide 48 of 54 MPharm PHA112 Haemostasis WEEK 26 Regulation of Haemostasis Striking A Balance Primary & secondary haemostasis must be restricted to the local site of vascular injury The size of both the primary & secondary haemostatic plugs must be restricted – to keep blood vessel patent The fibrinolytic system must be regulated – to ensure removal of unwanted fibrin clots & preservation of fibrin in wounds Regulation achieved via combination of multiple endogenous antithrombotic & antifibrinolytic systems Slide 49 of 54 MPharm PHA112 Haemostasis WEEK 26 General Regulators of Haemostasis Physiological Regulator Endothelium Mechanism of action Global haemostasis: Physical barrier Primary haemostasis: Secrete platelet antagonists, e.g. nitric oxide, prostacyclin, ADPase Secondary haemostasis: Express thrombomodulin, a membrane protein receptor. Binds thrombin and activates protein C (inhibits the co-factors FVa, FVIIIa) and the endothelial protein C receptor (cofactor for protein C activation) Express surface heparin-like glycosaminoglycans (GAGs), which enhance ATIII & TFPI activity Fibrinolysis: Secrete fibrinolytic inhibitors, e.g. plasminogen activator inhibitor-1 & 2 (PAI-1, PAI-2) Plasma proteins (Protease inhibitors) Slide 50 of 54 Global haemostasis: Produced in the liver, e.g. α2-macroglobulin, α1-antitrypsin. Act as non-specific inhibitors of serum proteases (activated coagulation factor enzymes, plasmin, etc) MPharm PHA112 Haemostasis WEEK 26 Regulators of Secondary Haemostasis Physiological Regulator Mechanism of action Tissue factor pathway Produced in endothelial cells. Inhibits the TF-FVIIa & TF-FVIIaFXa complexes. Rapidly inhibits any free factor Xa generated by inhibitor (TFPI) the TF-FVIIa complex (initiation of phase of 2 haemostasis). Antithrombin III (ATIII) Produced in the liver. Inhibits multiple coagulation factors, FIIa, FXa, FXIIa, FIXa, FXIa, TF-FVIIa. Activity is enhanced by heparin (exogenous) & heparin-like GAGs on endothelial cells. Protein C Vitamin K-dependent plasma glycoprotein produced in liver. Inactivates Fva & FVIIIa. Activated by thrombin via binding to thrombomodulin on endothelial cells. Activation further amplified by binding of thrombomodulin-thrombin complex to the endothelial protein C receptor. Protein S Vitamin K-dependent and produced in the liver. Cofactor for both TFPI & activated protein C. Occurs as free or bound to an acute phase protein, C4-binding protein. Only free protein S is able to act as a cofactor for TFPI and activated protein C. Slide 51 of 54 MPharm PHA112 Haemostasis WEEK 26 Negative Regulation of the Coagulation Cascade (Secondary Haemostasis) Slide 52 of 54 MPharm PHA112 Haemostasis Regulators of Fibrinolysis WEEK 26 Physiological Regulator Mechanism of action Thrombin-activatable Carboxypeptidase B, produced in the liver. Binds to lysine fibrinolytic inhibitor residues in fibrin, preventing plasminogen binding and activation. Activated by the thrombin burst (amplification & (TAFI) propagation phases of secondary haemostasis). Plasminogen activator Produced by endothelial cells. Binds and inactivates tissue plasminogen activator. Inhibited by thrombin-thrombomodulin inhibitor-1 (PAI-1) complex. 2-Antiplasmin Binds and inactivates plasmin. Produced in the liver. Polyphosphates Released from dense granules in platelets during activation. Form a dense fibrin network, which resists lysis. Extracellular nuclear material (histones, DNA, high mobility box proteins) Binds to the fibrin network, inhibiting fibrin degradation, promotes PAI-1 inhibition of tPA. Slide 53 of 54 MPharm PHA112 Haemostasis WEEK 26 Haemostatic Pendulum Slide 54 of 54 MPharm PHA112 Haemostasis