Transcript Document

Hemostasis & Thrombosis
Beth A. Bouchard
BIOC 212: Biochemistry of Human Disease
Spring 2006
HEMOSTASIS
Hemorrhage
Thrombosis
Hemostasis
HEMOSTASIS (CONT.)
1). INITIATION
Vessel wall – endothelial cells and subendothelial components
2). LOCALIZATION
Platelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATION
Plasma coagulation proteins (factors)
4). TERMINATION
Plasma coagulation protein inhibitors
5). ELIMINATION
Fibrinolytic system
Vessel Wall: Endothelial cells
Antithrombotic Attributes of Vascular
Endothelium
Vessel Wall (cont.)
Response to Vessel Wall Injury:
Platelet adhesion
• Exposure of flowing blood and platelets to
subendothelial components
• Platelets bind to the subendothelial collagen bound
to von Willebrand factor (vWF), which is secreted
from endothelial cells directly into the
subendothelial space or adsorbed from plasma
following endothelial cell secretion
• vWf also binds directly to platelets via
glycoprotein Ib-IX
PLATELETS
Platelets adhered to damaged endothelium
Platelet Plug Formation = 1° hemostasis
Platelet Plug Formation: Platelet activation
• Activated via their interaction with
subendothelial collagen
• Additional platelet agonists include ADP,
epinephrine, thrombin, immune complexes,
and high shear stress – all of the compounds
interact with specific platelet membrane
receptors
• Several platelet activation pathways are
initiated
Platelet Plug Formation: Platelet
activation events
• Platelet shape change: extend
pseudopodia, which facilitates aggregation
and coagulant activity
• Release of alpha and dense granule
contents including a number of compounds
involved in hemostasis (eg ADP, factor V
and fibrinogen)
• Aggregation
Response to Vessel Wall Injury:
Vasoconstriction
• Temporarily reduces local blood flow and
hence, blood loss
• Mediated in part by serotonin and
thromboxane A2 (TXA2) from activated
platelets
Serotonin is released from platelet dense granules
TXA2 is a product of platelet prostaglandin metabolism
Activated platelets
Platelet Plug Formation: Platelet
aggregation
• Platelet activation results in the functional
expression of membrane receptors normally
expressed in a non-functional state (glycoprotein
IIb-IIIa)
• Fibrinogen from the plasma or released from
activated platelet alpha-granules binds to
activated glycoprotein IIb-IIIa membrane
receptors effectively bridging platelets to each
other
Platelets adhered to and aggregated upon
collagen
Platelet Plug Formation
HEMOSTASIS
1). INITIATION
Vessel wall – endothelial cells and subendothelial
components
2). LOCALIZATION
Platelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATION
Plasma coagulation proteins (factors)
4). TERMINATION
Plasma coagulation protein inhibitors
5). ELIMINATION
Fibrinolytic system
BLOOD COAGULATION
BLOOD COAGULATION (CONT.)
• Deficiencies in all of the factors, except factor
XII, lead to a bleeding tendency in the affected
individual
• Described as a ‘waterfall’ or ‘cascade’ sequence of
zymogen (pro-enzyme) to enzyme conversions, with
each enzyme activating the next zymogen in the
seqeunce
• Activated factor enzymes are designated with an
“a”, e.g. factor Xa
Common constituents of coagulation
complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Protein cofactor
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Common constituents of coagulation
complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Protein cofactor
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Functional Domains of the Vitamin Kdependent Zymogens
Gamma (g)-carboxyglutamic acid
Formation of Gla residues subsequent to
protein synthesis (post-translational)
H2
•
Group of related, fat soluble compounds, which differ in the
number of side-chain isoprenoid units
•
Plant derived (vitamin K1) and synthesized by intestinal bacteria
(vitamin K2)
Common constituents of coagulation
complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Protein cofactor
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Prothrombin
-Thrombin
Prothrombinase
Components
FXa
Ca2+FXa
2+
FVa Ca
HC
Ca2+ FXa
FVa HC 2+
Ca
FVa LC
Relative Rate
of Prothrombin
Activation
1
300,000
FVa LC
Ca2+ FXa
Ca2+
30
Prothrombinase
Ca2+ FXa
FVa HC
FVa LC
Relevance of complex formation and its constituents
300
Common constituents of coagulation
complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Protein cofactor
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
** Express anionic phospholipids and membrane receptors
for coagulation proteins.
In platelets, the expression of this membrane surface is activation-dependent.
Activate platelets
HEMOSTASIS (CONT.)
1). INITIATION
Vessel wall – endothelial cells and subendothelial components
2). LOCALIZATION
Platelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATION
Plasma coagulation proteins (factors)
4). TERMINATION
Plasma coagulation protein inhibitors
5). ELIMINATION
Fibrinolytic system
INHIBITORS
INHIBITORS (cont.)
FIBRINOLYSIS
FIBRINOLYSIS (CONT.)
Platelet Plug Formation
• Measured clinically as the bleeding time
• Normal bleeding time is from 2 – 10 min
• Usually the bleeding time is sufficient to
detect defects of platelet adhesion and
aggregation, in which it is prolonged
Intrinsic Pathway of Blood Coagulation
• No factors extrinsic to the blood are involved
• Clinical test to assess the functionality of this
pathway is the activated partial thromboplastin
time (aPTT)
– Kaolin and cephalin are added to the test plasma sample
– The normal range is ~30 – 50 seconds (varies slightly
depending on the laboratory)
– Prolongations in the aPTT are observed in deficiencies of
factors XI, IX, VIII, X, and V, prothrombin, or
fibrinogen.
– Used to test for common congenital hemophilias
(deficiencies in IX, VIII, or XI) and to monitor heparin
treatment
Extrinsic Pathway of Blood Coagulation
• Extrinsic refers to tissue factor, which is
expressed on subendothelial cells
• Clinical test to assess the functionality of this
pathway is the prothrombin time (PT)
– Lipidated tissue factor is added to test plasma sample
– The normal range is ~10-15 seconds (varies slightly
depending on the laboratory)
– Prolongations in the PT are observed in deficiencies of
factors VII, X, V, prothrombin, or fibrinogen.
– Used to test for the rare congenital deficiencies in these
factors: More often it is used to diagnose acquired
bleeding disorders resulting from vitamin K deficiency,
oral anticoagulants (e.g. warfarin), and liver disease
Thrombin Time (TT)
In this test, thrombin is added to plasma
– The normal range is ~10-15 seconds (varies
slightly depending on the laboratory)
– Prolongations in the TT are observed in
congenital fibrinogen deficiency or acquired
fibrinogen deficiency resulting from
consumption of fibrinogen in DIC (disseminated
intravascular coagulation), or may occur
following treatment with fibrinolytic drugs
Bleeding disorders can span the spectrum from weeping
blood vessels to full-fledged internal and external
hemorrhage
Hemorrhage
Genetic defects:
platelet abnormalities
blood vessel wall abnormalities
clotting factor deficiencies (hemophilias)
excess clot breakdown (fibrinolysis)
Acquired defects:
liver disease (site of clotting factor synthesis)
vitamin K deficiency
autoimmune disease (platelet destruction)
trauma
Disorders of Platelet Adhesion or
Aggregation
• Affecting constituents of the vessel wall
• Affecting the ability of the platelet to
interact with the subendothelium at sites
of blood vessel injury
• Affecting the ability of the platelet to
interact with other platelets
Vessel Wall Defects
• von Willebrand’s disease: a group of autosomal
dominant disorders that result in reduced or
abnormal synthesis of vWF
• Defects in collagen synthesis
Ehlers-Danlos Syndrome: congenital defect in
collagen synthesis
Scurvy: results from vitamin C (ascorbic acid)
deficiency, which is involved in collagen synthesis
Excess exogenous or endogenous corticosteroids: also
leads to acquired deficiency in collagen synthesis
Platelet Defects
• Bernard-Soulier Syndrome: expression of
low levels of or defective glycoprotein IbIX on the platelet surface
• Glanzmann’s thrombasthenia: expression
of low levels of or defective glycoprotein
IIb-IIIa on the platelet surface
Other Defects
• Fibrinogen: deficiency of or production of
abnormal protein
• Acquired disorders include low platelet
count (thrombocytopenia) as a result of
defective formation of platelets by the
bone marrow or excessive destruction of
platelets
Bleeding disorders can span the spectrum from weeping
blood vessels to full-fledged internal and external
hemorrhage
Hemorrhage
Genetic defects:
platelet abnormalities
blood vessel wall abnormalities
clotting factor deficiencies (hemophilias)
excess clot breakdown (fibrinolysis)
Acquired defects:
liver disease (site of clotting factor synthesis)
vitamin K deficiency
autoimmune disease (platelet destruction)
trauma
Vitamin K deficiency
• Deficiency of vitamin K is rare because of its wide
distribution in nature, and its production by intestinal
bacteria
• Found in individuals with liver disease and fat malabsorption
- it is associated with bleeding disorders
• Newborn infants (especially preemies) are also at risk
- Placenta is insufficient in the transfer of maternal vitamin K
- Concentration of circulating vitamin K drops immediately after
birth, and it recovers upon absorption of food
- Gut of the newborn is sterile
Thus, newborns are given an injection of vitamin K following
birth.
Hemophilias A and B
• Hemophilias A and B are cause by deficiencies in
factors VIII or IX, respectively
• Affect ~1 in 10,000 males
• Inherited as a recessive X-linked trait (Mom would
be an unaffected carrier)
• Treated by administration of factor VIII or
factor IX concentrates
• Recombinant factor VIII or XI
• Gene therapy trials
Bleeding disorders can span the spectrum from weeping
blood vessels to full-fledged internal and external
hemorrhage
Hemorrhage
Treated by factor replacement
Thrombosis can be manifested as a transient, short-term
or episodic event in individuals with chronic or recurring
clotting. It is the major cause of both stroke and heart
attacks.
Thrombosis
Genetic defects:
clotting factor INHIBITOR deficiencies
decreased fibrinolysis
Acquired defects:
atherosclerosis
Pharmacologic Approaches to Prevent
Thrombosis
Antiplatelet agents - block activation,
aggregation or intraplatelet agonist synthesis
Effective anticoagulant therapy includes both
antiplatelet and antithrombin agents
Antiplatelet Drugs
Blood “thinners” - coumadin (warfarin): inhibition of “gla” formation
in the liver
Coumadin blocks reformation of reduced vitamin K, which essentially
stops the post-translational modification of the glutamic acid residues
at the amino-termini of the VKDP’s, since vitamin K is oxidized during
the reaction.
Blood “thinners” - heparin: potent cofactor for ATIII-catalyzed
inhibition of procoagulant serine proteases
Snakes
Leeches
Blood-sucking Insects