Von Willebrand Factor (vWF) & vWD

Von Willebrand Factor (vWF)

• vWF is a large glycoprotein involved in hemostasis (blood clotting) with a half-life of 24 hours.
• The mean plasma concentration of vWF is 1 mg/dL
• Synthesized mainly by endothelial cells and megakaryocytes
• Stored in Weibel-Palade bodies (endothelial cells) and α-granules (platelets)
• Binds platelets to exposed subendothelial collagen at sites of vascular injury through GP Ib/IX/V, its primary surface receptor.
• Functions include platelet aggregation, where it bridges platelets together via glycoprotein (GP) Ib and GP IIb/IIIa receptors
• It acts as a carrier for Factor VIII, protecting it from degradation and prolonging its half-life.
• Largest multimeric protein (600,000-20,000,000 Da) composed of multiple subunits.
• vWD is the most common inherited bleeding disorder.
• Caused by a deficiency or dysfunction of vWF.
• TTP involves excessive vWF multimers due to ADAMTS13 enzyme deficiency
• Leads to microvascular thrombosis.
• Lab evaluation can be done using vWF Antigen (vWF:Ag)
• Measures amount of vWF in plasma.
• Assess VWF function in platelet binding, via vWF Activity (Ristocetin Cofactor Assay).
• Evaluates vWF's role in stabilizing Factor VIII, through Factor VIII Assay.

Dense Tubular System (DTS)

• A condensed remnant of the rough endoplasmic reticulum.
• Sequesters Ca2+, a key regulator of platelet activation via Calcium Storage
• Contains enzymes essential for producing platelet activation mediators, via Enzyme Support for Platelet Activation.
• Enzyme produces arachidonic acid for thromboxane A2 synthesis, via Phospholipase A2,.
• Cyclooxygenase (COX) converts arachidonic acid to prostaglandins, a precursor of thromboxane A2
• Thromboxane Synthetase converts prostaglandins into thromboxane A2, promoting platelet aggregation.
• Phospholipase C produces inositol triphosphate (IP3) and diacylglycerol (DAG), amplifying intracellular signaling.

Adhesion Receptor

• Adhesion is essential to blood vessel walls during hemostasis.
• This process absorbs plasma proteins for transport and storage.

Glycocalyx

• Surface layer of the platelet membrane (20–30 nm thick).
• Functions in adhesion
• The process adheres cells to blood vessel walls during hemostasis.
• Absorbs plasma proteins for transport and storage.
• Maintains a negative surface charge, preventing unintended aggregation.

Granule Types

• α-Granules: Have 50-80 per platelet, appear gray in electron microscopy, released via SCCS, and function in platelet adhesion and coagulation.
• Dense Granules: Have 2-7 per platelet, are black (opaque) in electron microscopy, release directly into plasma for roles in platelet activation and recruitment.
• Lysosomes: Have few per platelet, are 300 nm and enzyme-positive, released internally, and involved in matrix digestion and autophagy.

Intrinsic Tenase Complex

• Activation in vitro occurs when factor XII is exposed to a negatively charged surface.
• e.g., glass or kaolin activating the contact phase of coagulation.
• High-molecular-weight kininogen (HMWK) and prekallikrein activate XII into XIIa and XI.
• Factor XIa, with Ca2+, activates Factor IX to IXa.
• Factor IXa, platelet phospholipid, Ca2+, and Factor VIIIa convert Factor X to Xa.
• The intrinsic and common pathways are evaluated by the PTT test.

Thromboxane A2 (TXA2) Formation

• Thromboxane synthetase converts PGH2 into thromboxane A2 (TXA2)
• TXA2 binds to TP receptors (TPα or TPβ).

Protein C Activation

• Protein C Activation: A vitamin K-dependent, 62-kDa enzyme acting as an inhibitor.
• Thrombin and thrombomodulin on endothelium convert Protein C to Activated Protein C (APC).
• It enhances activation if Protein C binds to endothelial protein C receptor (EPCR).

α2-Antiplasmin & Plasminogen Activator Inhibitor-1 (PAI-1)

• They regulate fibrinolysis by inhibiting plasmin and plasminogen activation.

Vitamin K-Dependent Factors

• This involves factors 2, 7, 9, and 10
• Are essential for the clotting cascade.
• They bind to Gla residues
• Are chelated by citrate/oxalate/EDTA in anticoagulated blood.

Warfarin

• Warfarin affects Stable Factor (Proconvertin, Serum Prothrombin Conversion Accelerator)
• This factor is Vitamin K-dependent
• Deficiency in these factors causes bleeding disorders
• Functions through activation by Tissue Factor
• Shortest half life

Factor 8

• Without Factor VIII, Cofactor IXa causes Hemophilia A; circulating while bound to VWF and is unstable if free form
• Factor VIII:C - refers to procoagulant portion
• Factor VIII: Ag - refers to its antigenic properties.
• Factor VIIIR: RCo - platelet aggregation, as a response to ristocetin

Desseminated Intravascular Coagulation (DIC)

• "Dessiminated" likely refers to DIC, a condition involving abnormal blood clotting throughout the body's vessels.

Fibrin-Stabilizing Factor (Factor XIII)

• Fibrin-Stabilizing Factor cross-links fibrin to stabilize the clot.
• Fibrinoligase is activated by thrombin
• Deficiencies of Factor XIII lead to poor wound healing

Calcium (Factor IV)

• Calcium is essential for the clotting cascade.
• Calcium binds to Gla residues of Vitamin K-dependent factors
• Chelated by citrate/oxalate/EDTA in anticoagulated blood.

Tissue Factor (Factor III)

• Initiates the extrinsic pathway.
• Is expressed by endothelial cells and monocytes.
• Binds Factor VII to activate it.
• Only enters the plasma after vascular injury.
• There is no Factor III deficiency

Hemophilia A

• Deficiency in Factor VIII

Factor V Leiden

• Cofactor for factor Xa in prothrombinase complex with inactivation from Protein C.

Heparin

• Includes a Heparin-binding site (amino terminus) that enhances inhibition. Binds and inhibits target enzymes with its Reactive site (Arg329-Ser394)

Coagulation in vivo

• Involves cell-based processes that integrate cellular and biochemical events.
• Its goal is to maintain blood fluidity, prevent excessive bleeding, and support healing through tissue factor-expressing cells and platelets.
• Initiation begins with extrinsic tenase complex (TF: VIIa) activating factors IX and X, leading to a little thrombin generation (3%-5%).
• Propagation occurs on platelet surfaces, with intrinsic tenase (IXa: VIIIa) and prothrombinase (Xa: Va) complexes amplifying thrombin production (>95%).
• Thrombin converts fibrinogen to fibrin, stabilizes the clot via factor XIII, activates regulatory pathways to control clot formation.
• Coagulation is localized at points of injury because of tissue factor cells and active platelets.

HMWK

• High-Molecular-Weight Kininogen serves as a cofactor for the activation of Factor XII and prekallikrein.
• HMWK involved in the intrinsic coagulation pathway and inflammation.

Thrombomodulin

• It is thought to be part of a complex or system that regulates coagulation.

Labile Factor (Factor V)

• Cofactor for Factor Xa in prothrombinase complex and inactivated by Protein C.
• Deficiency leads to parahemophilia, Factor V Leiden.

Proetin C

• A vitamin K-dependent, 62-kDa enzyme acting as an inhibitor."
• This factor is key in Protein C Activation.

Proetin Z

• Is a Z-Dependent Protease Inhibitor (ZPI)
• Inhibits Factor Xa with Proetin Z, phospholipid and calcium.
• The inhibitory effect is boosted by Protein Z, at 2000 fold

Extrinsic Pathway

• Occurs whenever vascular endothelial cell sustains Injury and resulting in exposure of TF (Transmembrane receptor Tissue Factor).
• TF and Plasma Factor VII will connect to develop TFF VII-Ca complex.
• As along with Cat activates Factor X to Xa in the common pathway