Platelets and Integrins

Question 1

Platelet procoagulant activity

Answer 1

• platelets produce thrombin
• acts on PAR1/4 to promote coagulation
• PAR1 antagonists prevent this
• Scott syndrome = thrombin not synthesised
• anticoagulants prevent thrombin production

Question 2

Integrin activation - talin

Answer 2

• talin - binding of talin F3 binding site to the membrane proximal region o f beta3
• conformational change transmitted to extracellular domains
• blood cell - selective KO in mice = loss of aggregation (doesn’t represent human disease state as we are injuring mice)

Question 3

Molecular mechanism of integrin activation

Answer 3

• GPCR activates Rap1b
• activates RIAM
• talin binds beta 3
• integrin activation

Question 4

Integrin activation - Rap1b

Answer 4

• Rap1b = GTPase (on when bound to GTP)
• KO in mice platelets = partial but not complete loss of aggregation
• whereas talin KO = complete loss
• Rap1b KO completely blocks occlusion so partial loss of aggregation enough to cause complete prevention of occlusion

Question 5

Integrin activation - RIAM

Answer 5

• RIAM critical in cell lines but completely useless in platelets
• is cell line work wrong?
• or is there redundancy and something else takes on its role when there is a KO

Question 6

Rap1b activation

Answer 6

Rap1b-GDP —-> Rap1b-GTP

addition of GTP, release of GDP
carried out by GEFs (on switch)
activated by calcium

Rap1b-GDP<—-Rap1b-GTP

release of Pi
carried out by GAPs (Rasa3)
activated by P2Y12, GI, PI3K

GAPs
- off switch (rasa3)
- has a ph domain
- activated by beta-gamma of Gi