Clotting

Question 1

What is the function of platelets?

Answer 1

• Adhesion
  – Stick to damaged tissue
  – Sub-endothelial collagen
• Shape Change & Spreading
  – Pseudopodia, filopodia, lamellipodia
  – Patch up damaged tissue
• Release Reaction
  – Release stored mediators from granules
  • Dense granules: ADP, ATP, 5-HT etc…
  • Alpha granules: fibrinogen, αIIbβ3 etc…
  – Synthesise mediators de novo
  • e.g., TxA2 from arachidonic acid (via COX)
• Aggregation
  – Stick to each other
  – Fibrinogen crosslinks platelets (via αIIbβ3)
• Promote Coagulation
  – Surface phosphatidylserine
  – → thrombin & fibrin generation

Question 2

What activates platelets?

Answer 2

• Collagen – extra-cellular matrix
• Thrombin – coagulation cascade
• ADP – released from dense granules
• Thromboxane A2 (TxA2) – synthesised de novo
• Platelet Activating Factor (PAF) – lipid mediator synthesised de novo
• Adrenaline – circulating hormone
• Serotonin (5-HT) – released from dense granules

Question 3

What is the response to injury?

Answer 3

1. Loss of the endothelial layer exposes underlying collagen in the basement membrane (Type IV) and in deeper tissues (Types I & III)
2. Platelets adhere to the exposed collagen and become activated. They change shape and spread over the damaged area
3. The activated platelets release ADP from dense granules and synthesise TxA2. Platelets arriving at the site of injury are activated
   and recruited by ADP and TxA2
4. Platelet activation converts integrin aIIbb3 from a low to a high affinity conformation and it binds fibrinogen. This results in platelet aggregation and the formation of a primary haemostatic plug or thrombus

Question 4

What is integrin?

Answer 4

• Aka: GPIIb/IIIa (CD41/CD61)
• Heterodimers – α & β subunits
• Fibrinogen receptor
• Only expressed on platelets

Question 5

What is fibrinogen?

Answer 5

• Heterohexamer
  – 2 × α chains: 831 resid (-16)
  – 2 × β chains: 447 resid (-14)
  – 2 × γ chains: 427 resid
• Synthesised by liver and found in plasma in platelet alpha granules
• Two distinct binding motifs for αIIbβ3 – RGD motifs
• On activation, αIIbβ3 changes to high affinity form and fibrinogen binds
• Fibrinogen crosslinks activated platelets by binding to αIIbβ3. More platelets recruited to form a large aggregate