PREFINAL LECTURE L1: PLATELET PRODUCTION, STRUCTURE AND FUCNTION Flashcards
T or F:
Platelets are nucleated blood cells
F
They are non-nucleated
Average platelet count
150-400 x10^9/L
T or F:
Platelets count is slightly higher in women than in men
T
Platelet count is slightly lower in both sexes who are older than how many years of age
65 years
When does platelet trigger primary hemostasis
Upon exposure to sub-endothelial collagen or endothelial cell inflammatory proteins
Platelets arise from what bone marrow cells
Megakaryocytes
Largest bone marrow blood cells
Megakaryocytes
What do you call the characteristic of megakaryocytes wherein they posses multiple chromosome copies
Polyploid
Size of megakaryocytes
30-50 um in diameter
Observable morphology of megakaryocytes under Wright stain
Multilobulated nucleus, and abundant granular cytoplasm
Megakaryocytes account for how many percent of all bone marrow cells
Less than 0.5%
How many megakaryocytes on a normal Wright stained bone marrow aspirate smear may be identified
2-4 megakaryocytes per 10x LPF
A specialized microenvironment near blood vessels (especially venous sinusoids) where stem cells and other cells interact.
vascular niche
wide, thin-walled blood vessels in the bone marrow that help with blood cell movement into the bloodstream.
venous sinusoids
What causes megakaryocytes and HSCs to cluster in vascular niches?
cytokines from stromal cells
growth factor that recruits megakaryocyte progenitors
thrombopoietin (TPO)
megakaryocyte progenitors are recruited from
common myeloid progenitors
T or F:
Megakaryocytes can only be found in the bone marrow
F
can also be found in the lungs
megakaryocyte progenitors arise from the common myeloid progenitor under the influence of what transcription gene product
GATA-1
GATA-1 stands for
globin transcription factor-1
GATA-1 is a protein product of what gene
X chromosome gene GATA1
GATA-1 is regulated by what cofactor
FOG1
FOG1 stands for
friend of GATA
FOG1 is a product of what gene
zinc finger protein multitype 1 (ZFPM1) gene
megakaryocyte differentiation is suppressed by what transcription gene product
MYB
purpose of GATA-1 and MYB
balance megakaryocytopoiesis with erythropoiesis
how many megakaryocyte lineage-committed progenitor stages are there
3
enumerate the stages in order of differentiation
1) burst-forming unit (BFU-Meg)
2) colony-forming unit (CFU-Meg)
3) light density CFU (LD-CFU-Meg)
all three progenitor stages resemble what cell
lymphocytes
T or F:
the three progenitor stages can be distinguished by Wright-stained light microscopy
F
progenitor stage/s that are diploid and undergo normal mitosis
1) BFU-Meg
2) CFU-Meg
how many colonies can BFU-Meg form in culture
hundreds
how many colonies can CFU-Meg form in culture
dozens
progenitor stage/s that undergo endomitosis
LD-CFU-Meg
what makes endomitosis unique to megakaryocytes
DNA replication and cytoplasmic maturation are normal but cells lose their capacity to divide
3 factors that stimulate megakaryocyte formation from hematopoietic stem cells
1) TPO
2) Meg-CSF
3) IL-3
endomitosis is a form of mitosis that lacks what cell division processes
telophase and cytokinesis
what happens in cytokinesis
separation into daughter cells
As GATA-1 and FOG1-driven transcription slows, what transcription factor mediates the switch from mitosis to endomitosis
RUNX1
how does RUNX1 mediates the switch from mitosis to endomitosis
by suppressing the Rho/ROCK signaling pathway
what happens in reduced Rho/ROCK signal
actin and myosin levels will be inadequate
what happens if actin and myosin levels are inadequate
cytokinesis will fail
what transcription factor promotes DNA replication without cell division (endomitosis)
NF-E2
What does it mean for a cell to be polyploid (e.g., 8N, 16N, 32N)?
A polyploid cell has multiple complete sets of chromosomes
For example, 8N means the cell has four times the normal 2N DNA content
what does 128N indicate
this level of ploidy is unusual, and may signal hematologic disease
T or F:
some megakaryocytes reach 128N
T
purpose of employing multiple DNA copies of megakaryocytes
to synthesize abundant cytoplasm
phase in which microscopists become able to recognize the unique Wright-stained morphology of megakaryocytes
terminal differentiation
least differentiated megakaryocyte precursor
MK-1 stage
MK-1 is aka
megakaryoblast
T or F:
megakaryoblast look like lymphocytes
F
they no longer do
megakaryoblasts are not reliably distinguishable with what cells
bone marrow myeloblasts
Vague clues that help distinguish megakaryoblasts
1) plasma membrane blebs
2) blunt projections from the margin
the megakaryoblast begins to develop most of what cytoplasmic ultrastructures
1) procoagulant-laden-α-granules
2) dense granules
3) demarcation system (DMS)
series of membrane-lined channels that invade from the plasma membrane and grown inward to subdivide the entire cytoplasm
DMS
DMS is biologically identical to what
megakaryocyte’s plasma membrane
what does DMS do during thrombocytopoiesis
delineates the individual platelets
developmental stage of megakaryocyte where the nucleus begins to indent and the cell has 4N DNA content
MK-II stage
MK-II is aka
promegakaryocyte
when does the promegakaryocyte reaches its full ploidy level
by the end of MK-II stage
most abundant megakaryocyte stage
MK-III
MK-III is aka
megakaryocyte
describe the morphology of megakaryocytes (nucleus, chromatin, cytoplasm)
1) nucleus is intensely dented/lobulated
2) variably condensed chromatin with light and dark patches
3) azurophilic and granular cytoplasm
platelet shedding is aka
thrombocytopoiesis
what makes megakaryocytes “platelet-like”
because of the spread of the DMS and α-granules
during thrombocytopoiesis, a single megakaryocyte may shed how many platelets
2000-4000 platelets
in an average size healthy human there are how many megakaryocytes, producing how many platelets per day
10 megakaryocytes producing 10 platelets per day
total platelet population turn over
8 to 9 days
at what instances where platelet production may rise by as much as tenfold
increased platelet consumption e.g. immune thrombocytopenic purpura
evidence of platelet budding or shedding in megakaryocyte cultures
1) DMS dilates
2) longitudinal bundles of tubules form
3) proplatelet process develop
4) transverse constrictions appear throughout the proplatelet process
how does proplatelet process shed platelets
they pierce through or between sinusoid-lining endothelial cells and extend to the venous blood
thrombocytopoiesis leaves behind naked megakaryocyte nuclei to be consumed by
marrow macrophages
what methods are used to identify visually indistinguishable megakaryocyte progenitors in hematologic disease
1) immunostaining of fixed tissue
2) flow cytometry with immunologic probes
3) FISH with genetic probes
what are the megakaryocyte membrane markers
1) MPL
2) CD34
3) CD41
4) CD61
5) CD36
6) CD42
7) CD62
MPL stands for
myeloproliferative leukemia protein
what is MPL
TPO receptor site
T or F:
MPL is present at all maturation stages
T
CD34 is aka
stem cell and myeloid progenitor marker
T or F:
CD34 disappears as differentiation proceeds
T
marker that first appears in megakaryocyte progenitors
CD41/CD61
CD41/CD61 is aka
glycoprotein (GP) IIb/IIIa
T or F:
CD41/CD61 remains present throughout maturation
T
proteins that may be detected in the fully developed megakaryocyte
1) cytoplasmic coagulation factor VIII
2) VWF
3) fibrinogen
weight of TPO
70,000 Dalton
TPO possesses how many percent homology with erythropoietin
23%
mRNA for TPO has been found in what organs
kidney, liver, stromal cells, and smooth muscle cells
primary source of TPO
liver
T or F:
TPO is a hormone
T
circulates as a hormone in the plasma
aside from being a hormone, TPO is also a
ligand
how does TPO function as a ligand
binds the megakaryocyte and MPL
define v-mpl
viral oncogene associated with murine myeloproliferative leukemia
T or F:
The plasma concentration of TPO is directly proportional to platelet and megakaryocyte mass
F
inversely proportional
induces thrombocytopoiesis
TPO
elevate the platelet count in patients being treated for a variety of cancers, including acute leukemia
Synthetic TPO mimetics (analogs)
a non immunogenic oligopeptide that raises the platelet count in patients with chronic immune thrombocytopenic purpura (ITP)
romiplostim (Nplate, Amgen)
nonpeptide TPO mimetic that binds an MPL site separate from romiplostim and is used in the treatment of chronic ITP and patients with thrombocytopenia resulting from chronic hepatitis C or severe aplastic anemia
eltrombopag (Promacta, Novartis)
cytokines that function with TPO to stimulate megakaryocytopoiesis
IL-3
IL-6
IL-11
interleukin that acts in synergy with TPO to induce early differentiation of stem cells
IL-3
interleukins that act in presence of TPO to enhance endomitosis, megakaryocyte maturation, and thrombocytopoiesis
IL-6
IL-11
IL-11 mimetic that stimulates platelet production in patients with chemotherapy-induced thrombocytopenia
oprelvekin (Neumega, Pfizer)
cytokines and hormones that participate with TPO and interleukins
1) stem cell factor
2) GM-CSF
3) G-CSF
4) acetylcholinesterase-derived megakaryocyte growth stimulating peptide
stem cell factor is aka
kit ligand, mast cell factor
factors that inhibit in vitro megakaryocyte growth
1) Platelet factor 4 (PF4)
2) B-thromboglobulin
3) neutrophil-activating peptide 2
4) IL-8
reduction of these transcription factors diminish megakaryocytpoiesis at the progenitor, endomitotic, and terminal maturation phases
1) FOG1
2) GATA-1
3) NF-E2
platelets are distributed throughout the RBC monolayer at (value)
7-21 cells per 100 x field
size of platelets
2.5 um in diameter
the mean platelet volume is measured in
buffered isotonic suspension
normal MPV
8-10 fL
shape of circulating resting platelet
biconvex
shape of platelets in EDTA blood
round-up
average platelet count of men over 65
122-350 x 10^9/L
average platelet count of women over 65
140-379 x 10^9/L
proportion of platelets sequestered within the spleen
one third
appear in compensation for thrombocytopenia
reticulated platelets
reticulated platelets are aka
stress platelets
size of reticulated platelets
> 6 um
MPV of reticulated platelets
12-14 fL
shape of reticulated platelets in citrated tubes
cylindrical and beaded
how can platelet dense granule nucleotides interfere with the quantitative evaluation of reticulated platelet production
falsely raising by binding nucleic acid dyes
T or F:
reticulated platelets are potentially prothrombic
T
neutral phospholipids of plt plasma membrane that predominate the outer blood plasma layer
phosphatidylcholine
sphyngomyelin
anionic or polar phospholipids that predominate in the inner, cytoplasmic layer
phosphatidylinositol
phosphatidylethanolamine
phosphatidylserine
how can phosphatidylinositol support platelet activation
by supplying arachidonic acid
arachidonic acid becomes converted to
thromboxane A2
cellular and humoral stimuli that activates receptors
ligands or agonists
platelet membrane surface
glycocalyx
glycocalyx absorbs what substances
albumin, fibrinogen and other plasma proteins
glycocalyx absorbs plasma proteins and transports them to internal storage organelles with the use of what process
endocytosis
formed when the plasma membrane invades the platelet interior
surface-connected canalicular system (SCCS)
upon platelet activation, the SCCS is the route for
endocytosis and secretion of a-granule contents
condensed remnant of the rough endoplasmic reticulum
dense tubular system (DTS)
the DTS sequesters what molecules and enzymes that support platelet activation
1) calcium
2) phospholipase A2
3) cyclooxygenase
4) thromboxane A2
5) phospholipase C
6) inositol triphosphate (IP3)
7) diacylglycerol (DAG)
which enzyme/s support the production of thromboxane A2
1) PLA2
2) COX-1/2
3) thromboxane synthetase
which enzyme/s support the production of IP3 and DAG
phospholipase C
what maintains the platelet’s discoid shape
circumferential microtubules
tubules disassemble at refrigerator temp or when platelets are treated with
colchicine
shape of platelets when the tubules disassemble
round
platelets shape at 37C
disc shape
meshwork composed of actin
microfilaments
function of actin
contractile in platelets and anchors the plasma membrane glycoproteins and proteoglycans
actin appearance in the resting platelet
globular and amorphous
actin appearance when the cytoplasmic calcium concentration rises
filamentous and contractile
the cytoplasm contains intermediate filaments of
desmin and vimentin
how does the intermediate filaments contribute in maintaining the platelet shape
connect actin and the tubules
what control the plt shape change, extension of pseudopods, and secretion of granule contents
1) microtubules
2) actin microfilaments
3) intermediate microfilaments
dense granules and α-granules share some of the same key membrane proteins, such as:
P-selectin
αIIbβ3 (aka GPIIb/IIIa)
GPIb/IX/V complex
there are how many a-granules in each platelet
50-80
color of a-granules in osmium-dye transmission electron microscopy preparations
medium gray
a-granules contain what substances
proteins
as the platelet becomes activated, a-granule membranes fuse with
SCCS
what happens to a-granules during platelet activation
flow to the microenvironment and participate in plt adhesion and aggregation and support plasma coagulation
how many dense granules per platelet
2-7
T or F:
dense granules appear later than a-granules in megakaryocyte differentiation
T
color of dense granules under osmium
black (opaque)
what happens to dense-granules during platelet activation
migrate to the plasma membrane and release their contents directly into the plasma on platelet activation
aside from proteins, platelets also contain what substance similar to neutrophils
lysosomes
what are the platelet dense granules
1) calcium and magnesium
2) ATP
3) ADP
4) serotonin
property of ADP
supports neighboring platelet aggregation by binding to P2Y1 and P2Y12 ADP receptors
property of serotonin
vasoconstrictor that binds endothelial cells and platelet membrane
property of calcium and magnesium
support platelet activation and coagulation
platelet membrane contains what receptors
1) Cell adhesion molecule (CAM) integrin family
2) seven-transmembrane receptor family
3) miscellaneous receptors
purpose of integrins
allows platelet to adhere to the injured blood vessel lining by binding collagen
what does integrin being heterodimeric mean
it composed of two dissimilar proteins
an integrin that binds the subendothelial collagen that becomes exposed in the damaged blood vessel wall
GP Ia/IIa
Which integrin binds to laminin?
α6β1
which integrin binds to fibronectin
α5β1
What platelet receptors promote adhesion by binding to endothelial proteins?
α5β1 and α6β1
It binds to collagen and the adhesive protein thrombospondin.
GP IV
What is GP Ib/IX/V composed of?
-Two GP Ibα
-two GP Ibβ
-two GP IX
-one GP V subunit
What is the function of GP Ib/IX/V under high shear conditions?
binds to von Willebrand factor (vWF), and support platelet tethering for capillaries and arterioles
What is the role of GP Ibβ in platelet signaling?
interacts with actin-binding protein, enabling “outside-in” signaling
What do GP IX and GP V contribute to the GP Ib/IX/V complex?
help assemble the four GP Ib molecules
Where are the integrin αIIbβ3 (GP IIb/IIIa) found in resting platelets?
1) plasma membrane
2) SCCS
3) a-granule membranes
What activates the integrin αIIbβ3 to bind fibrinogen?
Inside-out signaling triggered by agonist
Why is αIIbβ3 essential in platelet aggregation?
It binds fibrinogen, forming bridges between platelets, leading to aggregation.
Besides fibrinogen, which proteins bind to αIIbβ3
vWF, vitronectin, and fibronectin
amino acid sequence of the adhesive proteins as well as fibrinogen
arginine-glycine-aspartate (RGD)
what are the agonists for seven-transmembrane repeat receptors (STRs)
1) Thrombin
2) Thrombin receptor activation peptide (TRAP)
3) adenosine diphosphate (ADP)
4) epinephrine
5) serotonin
6) thromboxane A2
STR of thrombin
PAR1 and PAR4
STR of ADP
P2Y1 and P2Y12
STR of TXA2
TPa and TPb
STR of epinephrine
a2-adrenergic
STR of prostacyclin
IP
effect of P2Y1 signaling
-increase in intracellular Ca levels
-contributes to initial platelet activation
-shape change
-formation of small REVERSIBLE aggregates
effect of P2Y12 signaling
-decrease in cyclic adenosine monophosphate (cAMP) levels
-formation of IRREVERSIBLE platelet aggregates
what happens during binding of TPa and TPb to TXA2
produce more TXA2 from the platelet that activates neighboring platelets (recruiting)
what happens during binding of epinephrine and a2-adrenergic receptor
-inhibits adenylate cyclase
-reduction in cAMP
prostacyclin is a prostaglandin produced by what cells
endothelial cells
what happens during binding of prostacyclin with IP receptor
-increase cAMP
-inhibition of platelet activation
IP means
inositol phosphate
what are the intercellular adhesion molecules (ICAMs)
CD50
CD54
CD102
ICAMs function
play a role in inflammation and immune reaction
what mediates platelet-to-white- blood cell and platelet-to-endothelial cell adhesion
platelet-endothelial cell adhesion molecule (PECAM)
marker of PECAM
CD31
a low affinity receptor for the immunoglobulin Fc portion that plays a role in heparin-induced thrombocytopenia
FcyIIA
marker for FcyIIA
CD32
integrin that facilitates platelet binding to endothelial cells, leukocytes
P-selectin
marker for P-selectin
CD62
where is CD62 found
a-granule membranes of the resting platelet
where does CD62 migrate
via SCCS to the surface of activated platelets
common means for measuring in vivo platelet activation
P-selectin quantification thru flow cytometry
T or F:
platelet adhesion, aggregation and secretion often occur simultaneously
T
what is shear force
stress created in vessel walls as blood flows
shear force in venules and veins
500 s-1
shear force in arterioles and capillaries
5000 s-1
shear force in stenosed arteries
up to 40,000 s-1
in shear force that exceed 1000 s-1, platelet adhesion and aggregation require as define sequence of events which involves what key players
1) collagen
2) tissue factor
3) phospholipid
4) VWF
5) CAMS, ligands, activators
what happens when blood vessel wall is injured
- disrupts the collagen of the ECM
- damaged endothelial cells release vwf and adhere to sites of injury
molecular weight of vwf
500,000 to 20,000,000 Daltons
what happens to WVF under shear stress
It becomes thread-like and unrolls, exposing sites for weak binding to GP Ibα.
Which platelet receptor component binds to VWF during shear stress?
GP Ibα, a part of the GP Ib/IX/V complex.
What is the function of the reversible binding between VWF and GP Ibα?
It tethers and decelerates the platelet’s forward motion.
Which enzyme regulates the interaction between VWF and platelets?
ADAMTS13.
ADAMTS13 is aka
VWF-cleaving protease
What does ADAMTS13 do to VWF?
cleaves large VWF multimers into smaller, less active forms
secretes ADAMTS13
liver
what disrupts the temporary VWF-GPIba tethering reaction
GPVI comes in contact with the exposed ECM collagen
What happens when type I fibrillar collagen binds to GP VI on the platelet?
triggers internal platelet activation pathways, releasing TXA2 and ADP in an “outside-in” reaction
receptors involved in the “outside-in” reaction
TPa and TPb for TXA2
P2Y1 and P2Y12 for ADP
What does the “inside-out” reaction triggered by TXA2 and ADP lead to?
raises the affinity of integrin α2β1 for collagen
What is the combined effect of GP Ib/IX/V, GP VI, and α2β1 integrin on the platelet?
platelet become firmly affixed to the damaged surface
What happens to the platelet after it is firmly affixed to the damaged surface?
It loses its discoid shape and spreads.
What is exposed on the blood vessel wall after injury, in addition to collagen and VWF?
Tissue factor
tissue factor is from
subendothelial smooth muscle cells and fibroblasts
tissue factor triggers the production of
thrombin
How does thrombin activate platelets?
it cleaves PAR1 and PAR4
the further activation caused by thrombin generates what
collagen and thrombin activated (COAT) platelet
role of TXA2 and ADP in platelet activation
activate neighboring platelets, triggering inside-out activation of integrin αIIbβ3
What does the inside-out activation of integrin αIIbβ3 enable?
enables integrin αIIbβ3 to bind RGD sequences of fibrinogen and VWF
What does P-selectin from the α-granule membranes promote?
promotes the binding of platelets with leukocytes
What happens to platelets upon further activation and aggregation in terms of its shape
discoid to round, form pseudopods
Why do platelets form a syncytium during aggregation?
Because platelets exhaust their internal energy sources and membrane integrity is lost
what occurs during lost of membrane integrity of platelets during aggregation
phosphatidylserine flip to the outer layer
what is synyctium
massive clump of platelets
in primary hemostasis, arteries may end with the formation of what
white clot
white clot is primarily composed of
plt and vwf
what does white clot indicate (pathologic)
inappropriate plt activation ; pathologic basis for arterial thrombotic events
examples of arterial thrombotic events
1) AMI
2) peripheral artery dse
3) ischemic stroke
why does avidity of a2b1 and GPVI receptors increase risk of cardiovascular events
high avidity-> more binding sites for collagen and vwf-> inc plt adhesion/aggregation-> inc risk of abnormal clotting
secondary hemostasis is
coagulation
What combination of factors triggers secondary hemostasis
1) polar phospholipid exposure on activated plt
2) plt fragmentation with cellular microparticle release
3) secretion of a-granules and dense granules
primary hemostasis: white clot; secondary hemostasis: ?
red clot
red clot is composed of
fibrin and rbcs
role of red clot
essential in wound repair
white clot: inappropriate coagulation in arteries; red clot: ?
inappropriate coagulation in veins and venules
pathologic indications of red clot
deep vein thrombosis and pulmonary embolism
triggers the actin microfilament contraction in platelets
-outside in activation of plt through STR (ADP+P2Y12)
-GPVI
what happens when intermediate filaments contract
moves the circumferential microtubules inward-> compressing the granules
Where do the contents of the a-granules and lysosomes flow after platelet activation?
SCCS
Where does dense granules migrate and secrete their contents
plasma membrane
dense granules secrete
vasoconstrictors and platelet agonists
what does vasoconstrictors and platelet agonists do
amplify primary hemostasis
role of a-granule contents in platelet activation
large molecule coagulation proteins that participate in secondary hemostasis
How do platelets support coagulation at the cellular level?
by presenting phosphatidylserine, where complexes assemble
what complexes assemble at the phosphatidylserine
factor IX/VIII (tenase) and factor X/V (prothrombinase) complexes
role of calcium in platelet-mediated coagulation
secreted by dense granules, helps form tenase and prothrombinase complexes.
contents of a-granules
1) fibrinogen
2) factor V
3) factor VIII
4) VWF
role of a-granules
supports the action of tenase and prothrombinase
all a-granule proteins
1) platelet-derived growth factor
2) endothelial growth factor
3) transforming growth factor B
4) fibronectin
5) thrombospondin
6) platelet factor 4
7) B-thromboglobulin
8) plasminogen
9) plasminogen activator inhibitor-1
10) a2-Antiplasmin
11) protein C inhibitor
a-granule protein:
supports mitosis of fibroblasts and smooth muscle cells
1) platelet-derived growth factor
2) endothelial growth factor
3) transforming growth factor-B
a-granule protein:
adhesion molecule
1) fibronectin
2) thrombospondin
a-granule protein:
heparin neutralization, suppresses megakaryocytopoiesis
platelet factor 4
a-granule protein:
only found in platelet a-granules
B-thromboglobulin
a-granule protein:
fibrinolysis promotion
plasminogen
a-granule protein:
fibrinolysis control
1) plasminogen activator inhibitor-1
2) a2-Antiplasmin
a-granule protein:
coagulation control
protein C inhibitor
What triggers the formation of platelet microparticles?
triggered by an activating stimulus that increases the intracellular calcium concentration in platelets
How do elevated calcium levels inside platelets contribute to microparticle formation?
- inhibits asymmetric distribution of phospholipids
- activates enzyme calpain
what does caplain do
cleaves platelet cytoskeleton
inhibition of asymmetric distribution of phospholipids and caplain leads to
outward blebbing of the plasma membrane and formation of plt microparticles
most abundant microparticles in the circulation
plt microparticles
plt microparticles are formed after
exposure of plt to strong agonists or shear stress
How do platelet microparticles influence coagulation?
exposing phosphatidylserine on their surface, crucial for the assembly of tenase and prothrombinase
other functions of plt microparticles aside coagulation
1) modulate inflammation
2) oxidative stress
3) angiogenesis
4) thrombosis
elevated plt particles is significant in what conditions
hypercoagulable conditions
G-proteins are made up of what subunits
a, B, y (heterotrimers)
What happens to G-proteins when an agonist binds to a receptor?
The G-protein releases GDP and binds GTP, activating the G-protein.
What does the activated Gα subunit do?
briefly separates from the complex (G-protein), acts like an enzyme to hydrolyze GTP to GDP, and triggers signaling pathways
What are two major signaling pathways triggered by G-protein activation?
Eicosanoid synthesis and the IP3-DAG pathway.
how does hydrolysis done by G-protein leads to activation of pathways
hydrolysis resulted to phosphorylation
eicosanoid synthesis is aka
prostaglandin, cyclooxygenase, or thromboxane pathway
What enzyme releases arachidonic acid from the membrane?
Phospholipase A2
5,8,11,14-eicosatetraenoic acid is aka
arachidonic acid
arachidonic acid is the substrate for
cyclooxygenase
what does cyclooxygenase do
converts arachidonic acid to prostaglandin G2 and prostaglandin H2
enzyme that acts on prostaglandin H2 to produce TXA2
thromboxane synthetase
what happens when TXA2 binds membrane receptors TPa or TPb
- inhibits adenylate cyclase activity
- reducing cAMP concentrations, which mobilizes ionic
calcium from the DTS
what does the rising cytoplasmic calcium level cause
contraction of actin microfilaments producing platelet shape change and further platelet activation
What enzyme do endothelial cells use in place of thromboxane synthetase in the cyclooxygenase pathway?
prostacyclin synthetase
end product of the cyclooxygenase pathway in endothelial cells
prostaglandin I2 (PGI2)
prostaglandin I2 (PGI2) is aka
prostacyclin
what pathway does prostacyclin+IP receptor binding activate
IP3-DAG pathway
effect of prostacyclin on adenylate cyclase and cAMP
accelerates adenylate cyclase-> increased cAMP
prostacyclin effect on intracellular Ca levels
causes sequestration of ionic calcium to the dense tubular system (DTS), making calcium unavailable in the cytoplasm
what does the unavailability of Ca in the cytoplasm do to platelet function
shuts down plt function
endothelial cell eicosanoid pathway: suppress platelet activation; platelet eicosanoid pathway:
promotes plt activation
half-life of TXA2
30 seconds
TXA2 diffuses from the platelet and reduced to
thromboxane B2
What stable metabolite of thromboxane B2 is measurable in urine?
11-dehydrothromboxane B2
enzyme that initiates the IP3-DAG pathway in platelets
phospholipase C
What does phospholipase C do in the IP3-DAG pathway?
cleaves PIP2 to IP3 and DAG
function of IP3 in plt activation
releases calcium from the dense tubular system (DTS)
the release of Ca by IP3 causes what
actin microfilament contraction; activation of phospholipase A2
function of DAG in the IP3-DAG pathway
activates phosphokinase C
activation of phosphokinase C by DAG triggers the phosphorylation of what protein
pleckstrin
what does pleckstrin do
regulates actin microfilament contraction
Despite lacking nuclei, what allows platelets to synthesize proteins?
1) ribosomes
2) polyribosome complexes
3) regulatory factors
4) microRNAs (miRNAs)
5) template mRNAs
What triggers protein synthesis in platelets?
- platelet activation
- ligand binding to the GPIIb/IIIa receptor
What is the significance of platelet protein synthesis?
to alter their phenotype depending on level of activation
