Cellular elements of the haemostatic system. Platelets and von Willebrand factor. Flashcards
1
Q
Platelet Generation
A
- in bone marrow
- megakaryoblast ⇒ pro-megakaryocyte ⇒ immature megakaryocyte ⇒ megakaryocyte
- megakaryocyte undergoes endomitosis
- cytoplasmic differentiation produces platelets
- circulates 8-10 days
- result is anucleated, discoid cell, 2-4 mm diameter
2
Q
Platelet structure and function
A
- Peripheral zone: phospholipid membrane
- Sol-gel zone: cytoskeleton, microtubules - maintain shape
- Organelle zone: alpha/dense grnaule, lysosomes, mitochondria- stimulate adjacent platelets
- Membrane system: calcium storage, enzyme for prostaglandin synthesis
3
Q
Formation of Hemostatic plug
A
- Adhesion
- Secretion
- Aggregation
- Contraction
4
Q
Adhesion
A
- attachment of cells on a surface at the site of injury
- sub endothelial elements are exposed and platelets spread along the surface
- interaction with sub endothelium, vWF, GPIb
5
Q
Secretion/ Activation
A
- shape change when activated
- release of granular contents and prostaglandin metabolites
- these interact with receptors on adjacent platelets ⇒ incorporation into evolving aggregate
- activation by: ADP, collagen, epinephrine ⇒ PLA2 to release arachidoinic acid⇒ PGE2/H2⇒ thromboxane A2⇒ amplification
6
Q
Dense/delta granules
A
release of: ADP, serotonin, Ca2+
7
Q
Alpha granules
A
release of: platelet factor 4, b-thromboglobulin, platelet derived growth factor, vWF, factor V, thrombospondin, IgG and fibronectin
8
Q
Aggregation
A
- process by which platelets interact with one another to form a hemostatic plug
- GPIIb-IIIa complex binds vWF, undergoes Ca2+-dependent structural change and acts as receptor for fibrinogen
- fibrinogen allow bridging of activated platelets
- fibronectin, thrombospondin stabilize platelet aggregates
9
Q
Contraction
A
- initial hemostatic plug stops blood but is unstable and must be followed by fibrin clot
- platelets undergo contrictile response using actin-myosin apparatus
- actin binds GPIIb-IIIa, which is bound to fibrin ⇒ clot retraction
10
Q
Triggers for platelet activation
A
Thrombin Collagens Platelet activating factor ADP (autocrine/paracrine) Thromboxane A2 (autocrine/paracrine)
11
Q
vWF
A
- multimeric glycoprotein
- produced as ultra-large vWF in endothelium, megakaryocytes, sub endothelial CT
- stored in Weibel-Palade body
- Domains:
- D’/D3 domain: binds factor VIII
- A1 domain: binds GPIb-receptor, heparin, collagen
- A3 domain: binds collagen
- C1 domain: platelet integrin aiib B3
- cysteine knot domain: transformes TGFB and B-HCG
- function: bind to proteins, especially factor VIII, create contact between platelet and collagen + platelets with platelets
- no catalytic activity
- platelets have 2 types of vWF receptors: GP1ba, GPIIb/IIIa
12
Q
Receptors for platelet activation
A
- PAR1 receptor: thrombin binds, Gq coupled ⇒ initiation of platelet activation via contractile system, surface modification, TXA2 synthesis, granule release
- Adenosine, PGI2 ⇒ Gs ⇒ cAMP ⇒ inhibit platelet activation
- ADP ⇒ Gi ⇒ cAMP decrease
- receptors for collagen, PAF, fibrinogen, vWF, thromospondin, fibronectin, laminin
13
Q
GP1ba
A
- platelet receptor for vWF
- accessed all the time
- participates in initial adhesion, but weak interactions
- activated by shear stress when bound to vWF⇒ intracellular Ca2+ signal ⇒ ADP secretion
⇒ via ADP purinergic receptor ⇒ Ca2+ signal ampified ⇒ positive feedback (also activates GPIIb/IIIa
14
Q
GPIIb/IIIa
A
- belongs to integrin family
- only bind to vWF when activated (by GP1ba activation)
- stronger and more sustained adhesion
- vWF receptor + fibrinogen receptor
15
Q
P2Y receptor
A
- purinergic G-protein coupled receptor
- stimulated by ATP, ADP, UTP, UDP and UDP-glucose
16
Q
Ca2+ signal effects on platelets
A
- activates MLCK ⇒ contraction
- activates PLA2 ⇒ free arachidonic acid ⇒ TXA2 ⇒ platelet activation
- rearrangement of PM ⇒ activate scrambles enzyme ⇒ shuffling phospholipids for more negative outside ⇒ optimal for complexes to bind
- initiate secretion of vesicles in platelets
17
Q
Aspirin
A
- drug to prevent cardiovascular disease
- covalently modifies serine residue in COX1 ⇒ inhibits movement of arachidonic acid ⇒ no TXA2 ⇒ no platelet activation
18
Q
Neutrophils
A
- kill by NETosis
- process initiated by PAMP
- cells keep PM integrity but nucleus disintegrates + chromatin de-coondenses
- cell releases its DNA ⇒ DNA net in extracellular matrix
- nuclear + cell granule proteins bind to DNA net
- NET bind bacteria ⇒ exposed to cytotoxic contents
- NET DNA has strong negative charge ⇒ activator of FXII
- NET binds vWF to enhance platelet adhesion
- NET + fibrin⇒ resistant for fibrinolysis, deoxyribonucleases break down instead
- Neutrophil elastase: cleave TFPI + antithromin, removes kringle domain from plasminogen, inactivates a2-PI
19
Q
Endothelial cells: anti-coagulation
A
- platelets release: heparan sulfate, thrombomodulin, EPCR, TFPI
- endothelium release: NO⇒ s-nitrolates proteins’ free sh-group
20
Q
Endothelial cells: inhibition of platelets
A
- NO ⇒ soluble guanylyl cyclase ⇒ cGMP ⇒ PKG ⇒ inhibits phosholipase C + TXA2 receptor, activates MLCP
- cGMP inhibit PDE3 ⇒ increased cAMP ⇒ inhibits platelet activation
- activation of PGI2 receptor ⇒ cAMP ⇒ arachidonic acid attained ⇒ inhibition of platelet
- ecto-ADPase⇒ break down ADP ⇒ no positive feedback
21
Q
Endothelial cells: pro-fibrinolytic aspects
A
- endothelium produces tPA, uPA + prvides co-factor surface for plasminogen activation
- tPA ⇒ A2.p112 tetramer protein complex
- uPA receptor co-localise to protect uPA from PAI1
22
Q
Function of liver in hemostasis
A
- Hepatocytes: synthesis of proteins + post-translational modifications
- Kupffer cells: removal on inactive proteases
- Gla domain synthesis
23
Q
Membrane glycoprotein receptors
A
- GPIb-V-IX: bind to vWF immobilized on collagen
- GPIa/IIa: bind to collagen
- GP VI: bind to collagen
- GPIIb/IIIa: bind to free fibrinogen and vWF
24
Q
vWF binding sites
A
- collagen
- GPIb
- GPIIb/IIIa
- Factor VIII
- heparin
25
Q
ADAMTS13
A
- breaks down vWF
- a disintegrin and metalloprotease with a thrombospondin type 1 motif
