Lec. 48 Platelet Plug

Question 1

Two components to a thrombus

Answer 1

Platelet Plug: aggregation of platelets to plug the injury, called primary hemostasis

Fibrin Clot: coagulation factor proteases aggregate and produce thrombin, thrombin makes fibrin monomers that are cross linked to form a meshwork around the platelet plug, called secondary hemostasis

Question 2

Where are platelets produced?

Answer 2

Produced from megakaryocytes in bone marrow

Question 3

What is thrombocytopenia?

Answer 3

low platelet count

less than 1x10^5/mL

Question 4

What prevents resting platelets from adhering to each other or the vessel wall?

Answer 4

A negatively charged glycocalyx

Question 5

What organelles do platelets have and not have?

Answer 5

Platelets lack a nucleus, but they do have mitochondria and lysosomes.

Question 6

What granules do platelets have?

Answer 6

Dense granules: several per cell, store Ca2+ and ADP

Alpha granules: 50-80 per cell, store von Willebrand Factor, fibrinogen, coagulation factor V

Question 7

Two membranous structures that play an important role in platelet function

Answer 7

Canalicular system: invaginations in the plasma membrane of platelets, provides surface area for vesicular trafficking

Dense tubular system: network of ER in the platelet, stores Ca2+ and releases it during activation

Question 8

What is the function of glycoproteins in platelet adhesion, and what three types are there?

Answer 8

Glycoproteins are integrins that are essential for the binding of platelets to each other and the damaged endothelial wall.

1) GPIb: binds to von Willebrand Factor
2) GPIa & GPVI: bind to collagen
3) GPIIb/IIIa: bind to vWF or fibrinogen

Question 9

What happens to a platelet when it is activated?

Answer 9

Conformational changes in the cell surface receptors (glycoproteins)

Degranulation: granules are released into extracellular space, signals other platelets, facilitated by canalicular system

Change in cell shape: microtubules contract to change cell from disc shape to spiny spheres with pseudopods or filopodia

Question 10

Cell receptors on platelets bind to what ligands that activate the platelets?

Answer 10

Epinephrine
ADP
Thrombin
TXA2

Question 11

What ligands are released from platelets after they are activated?

Answer 11

Phospholipase A2 which results in the synthesis of TXA2

Degranulation releases platelet activators such as ADP

Question 12

How does GPVI activate platelets?

Answer 12

Binds collagen in the subendothelia and activates platelets through tyrosine-kinase coupled receptors.

Question 13

Interaction between Thrombin and PAR (protease activated receptor)

Answer 13

Thrombin is a protease that cleaves a small portion of a protein off the tail of a GCPR located on the surface of platelets called PAR. This tail becomes a tethered ligand that activates it’s own receptor and stimulateGalpha-q related signaling. This is a strong activator of platelets.

Question 14

What two molecules are in the subendothelial layer and are essential for the initial adhesion of platelets at the site of injury?

Answer 14

Collagen & vWF

Question 15

How does vWF tether platelets to collagen?

Answer 15

vWF can bind to both collagen and platelet integrin receptors (GPIb) to tether the two together.

Question 16

What platelet receptors can bind collagen directly?

Answer 16

GPIa, GPVI

Question 17

What are the binding sites for vWF?

Answer 17

FVIII, GPIb, Collagen, GPIIb/IIIa

Question 18

What is the relationship between endothelial cells, platelets, HMW vWF, and ADAMTS13?

Answer 18

Endothelial cells & platelets make and store HMW vWF which is stored and released from Weibel-Palade bodies in endothelial cells and alpha-granules in platelets.

HMW vWF is made by disulfide bonds formed between vWF monomers to form multimers.

In plasma, ADAMST13 is a protease that cleaves HMW vWF to smaller pieces that have less thrombogenic capacity.

Question 19

Describe the adhesion step of platelet plug formation.

Answer 19

Damaged endothelial cells expose collagen and vWF in subendothelial layer.

Resting platelets bind to the vWF through GPIb receptors.

GPIa&GPVI bind directly to the collagen to stabilize the adhesion.

Question 20

Describe events in primary activation/aggregation of platelet plug formation.

Answer 20

GPIa&VI bind to collagen in subendothelia

Signal transduction initiated by GPVI-collagen binding which activates the platelet through TKR signaling

Degranulation and release of TXA2

Binding of GPIa/Ib/VI cause conformational change in GPIIb/IIIa that allow it to bind well to vWF forming firm adhesion to vessel wall. (conformational change also allows it to bind well fibrinogen for secondary aggregation)

Question 21

What do GPIIb/IIIa bind to?

Answer 21

vWF and fibrinogen

Question 22

Describe events of secondary platelet activation.

Answer 22

Resting platelets in circulation become activated by TXA2, ADP, fibrinogen that were released from primary activation platelets when the underwent degranulation.

Question 23

Describe events of secondary aggregation.

Answer 23

Platelet-platelet binding through GPIIb/IIIa-fibrinogen to form a second layer of platelets bound at the site of injury.

Question 24

Relationship between shear forces and vWF

Answer 24

High shear forces cause a change in vWF that enhances its binding to GPIb.
High shear==>increased adhesion

Question 25

Describe the events that take place in the platelet activation, production of factors, and release of factors.

Answer 25

Platelet agonists (ADP, TXA2, Thrombin) bind to GCPR’s.
Gq-signaling causes increase in cytosolic Ca2+ levels.
Increased Ca2+ activates PLA2 which cleaves membrane phospholipids to create AA.
AA is converted into TXA2 through COX-1 and thromboxane synthase.
TXA2 is not stored, but transported out of cell.
Endoperoxides are intermediates in production of TXA2, they signal degranulation of alpha and dense granules which release platelet activators (ADP) and coagulation factors (Factor V, fibrinogen) into local area.

Question 26

How is thrombin produced?

Answer 26

Phosphatidyl serine is moved from the inner to outer membrane leaflet placing a neg charge on the outer surface.
Coagulation factors assemble on the surface.
Factors cleave each other to form Thrombin (a protease).
Thrombin is needed for primary and secondary hemostasis.
Thrombin activates resting platelets, and converts fibrinogen into fibrin.
Factor IIa=thrombin

Question 27

How do endothelial cells prevent clot formation?

Answer 27

Endothelial cells secrete prostacyclin PGI2
PGI2 binds to Gs-linked receptors on the platelets and cause increase in PKA and cAMP action
cAMP causes Ca2+ pump to move Ca2+ from the cytoplasm to the dense tubular system
Decreased Ca2+ prevents activation of PLA2 which prevents formation of TXA2 and results in no platelet aggregation, secretion, shape change, or adhesion

Question 28

Describe COX inhibitor drugs

Answer 28

Aspirin

Inhibits COX enzymes–> decrease in TXA2 production–>decrease platelet activation and degranulation

Question 29

Describe ADP-receptor drugs

Answer 29

Plavix
Inhibiting ADP receptors causes decrease in ADP-mediated secondary activation
Plavix is a non-competitive inhibitor

Question 30

Describe GPIIb/IIIa inhibitors

Answer 30

Abciximab (Reopro)
GPIIb/IIIa binds fibrinogen and anchors platelets to the vessel and to each other
Inhibiting them causes decrease in platelet aggregation

Question 31

Describe PGI2 agonists

Answer 31

Dipyridamole
Inhibits phosphodiesterases that break down cAMP
This causes increase in cAMP and less cytosolic Ca2+ and less PLA2 and less thrombosis

Question 32

Describe vWF deficiency

Answer 32

Most common inherited bleeding disorder
Caused by multiple mutations on vWF gene
Affects platelet adhesion and primary aggregation and clotting cascade because factor VIII levels are low
Presents with a range of bleeding tendency depending on severity of gene damage, asymptomatic to severe bleeding

Question 33

Describe TPP

Answer 33

Thrombotic Thrombocytopenia Purpura
Mutation in ADAMTS13 which prevents it from cleaving HMW vWF into smaller pieces
Causes exaggerated thrombosis forming microthrombi throughout the body
Thrombocytopenia occurs from more use of platelets in forming microthrombi
Symptoms: easy bruising (purpura), microthrombi throughout body, damage to RBC’s causing hemolytic anemia, damage to vessels which causes damage to kidney, heart, and brain

Question 34

Describe Bernard-Soulier syndrome

Answer 34

Defective gene for GPIb
Second most common disorder
Prevents adhesion of platelets to vWF to initiate platelet plug formation
Symptoms: slight thrombocytopenia, purpura, prolonged bleeding time, ABNORMALLY LARGE PLATELETS NEARING SIZE OF RBC’s

Question 35

Describe ITP

Answer 35

Idiopathic Thrombocytopenic Purpura
Autoimmune disease where body produces antibodies that bind to GP’s in platelets such as GPIb/IIb/IIIa and causes platelet degradation in the spleen.
Symptoms: severe thrombocytopenia, splenomegaly (more common in children than adults), bruising (purpura), superficial bleeding (petechial hemorrhages, red dots on the skin), gingival bleeding, prolonged bleeding

Question 36

Describe Glanzmann Thrombasthenia

Answer 36

Deficiency of GPIIb/IIIa
Prevents binding GPIIb/IIIa-vWF and fibrinogen so primary and secondary aggregation cannot happen
Symptoms: excessive bleeding, petechia and ecchymoses (purpura>1cm), prolonged bleeding time

Question 37

When there are low shear forces, how does the platelet bind to the subendothelium?

Answer 37

With low shear, the vWF doesn’t bind as well to GPIb, so the platelet uses GPIa/VI to bind directly to collagen.