CTB7: Cell biology of cardiovascular system Flashcards
What are larger blood vessels composed of?
Three layers:
intima
media
adventitia
Name the 6 parts of what blood vessels are composed of
Tunica adventitia External elastic lamina Tunica media Internal elastic lamina Tunica intima
What is the intima composed of?
Single-cell layer of endothelial cells which is smooth and non-adhesive for platelets and leukocytes
Discuss the arrangement of endothelial cells
Endothelial cells adhere tightly to each other, forming a selective barrier between blood and other components of the vascular wall. The endothelium covers a very large area in the body
What is the surface area of the lung endothelium?
130m^2
When do endothelial cells show a cobbled morphology?
cells tightly adhere to each other and their shape resembles cobblestones. Endothelial cells with cobblestone morphology can also be found in smaller blood vessels
When endothelial cells are in larger blood vessels what happens to their morphology/
In larger vessels, where blood flow is stronger, endothelial cells flatten out and elongate within the direction of flow, to minimise mechanical forces that act on their surface and so minimise the risk of vascular injury
Why do edges of the cells overlap?
To increase junctional area and strengthen barrier function
What environmental factors cause endothelial cells to synthesise or extract different vasoactive mediators?
Oxygen tension
Blood flow
Circulating cytokines
Growth factors
What vasoactive mediators does the endothelium synthesize or extract?
Endothelin-1 Norepinephrine Angiotensin 1 Thromboxane Prostacyclin (PGI2) Endothelial-derived relaxing factor nitric oxide (NO)
What does synthesis or extraction of vasoactive mediators do? (simplistic)
Translates information about environmental changes to the underlying smooth muscle cells, to control their reactivity and regulate vascular tone and initiate other adaptational changes that help maintain vascular homeostasis.
What are some of the functions of the endothelium?
regulation of vascular tone
barrier function
inflammatory responses
thrombosis and angiogenesis
barrier, regulating the exchange of fluids and nutrients between blood components and the surrounding tissues
In inflammatory conditions, the endothelium regulates trafficking of blood cells to the sites of injury
What is the media of blood vessels composed of?
vascular smooth muscle cells enmeshed in collagen and elastin, and this layer is separated from the intima by the internal elastic lamina.
What are the vascular smooth muscle cells of the media made up of? How does this help the role of the cell?
This collagen and elastin extracellular matrix forms a scaffold for the vascular smooth muscle cells and greatly contributes to the strength and compliance of the blood vessel wall.
What is the difference in structure between the pulmonary and aorta structure?
The aorta has many layers of smooth muscle in the media, with a great abundance of collagen and elastin in order to withstand arterial pressure.
The pulmonary artery, in contrast, has fewer layers of smooth muscle with relatively more elastin, making it suitable for large changes in volume without much change in pressure.
What is the main function of the vascular smooth muscle?
To regulate vascular tone by undergoing contraction and relaxation
What are the two types of regulation of contraction and relaxation of smooth muscles?
Endothelium-dependent or endothelium-independent
What does the adventitia consist of?
collagen, elastin, and other extracellular matrix interspersed with fibroblasts. The external elastic lamina is the layer of interwoven elastin fibrils between the adventitia and the media.
The adventitia, along with the external elastic lamina, contribute to the ________of a given blood vessel
compliance
What is the main function of adventitia fibroblasts?
To produce structural extracellular matrix proteins that regulate vascular compliance
Name two endothelium-derived vasorelaxants
nitric oxide (NO) and prostacyclin (PGI2)
Name an endothelium-derived vasoconstrictors
endothelin-1 (ET-1)
Where is nitric oxide produced from?
L-arginine by endothelial nitric oxide synthases
What is another product from L-arginine? (other than NO)
L-citrulline
What can endothelial nitric oxide synthases (eNOS) be activated by?
- shear stress
- increased O2 ventilation
- vasorelaxant substances e.g: bradykinin and acetylocholine
Where does NO diffuse into?
Underlying smooth muscle layer
What does NO do? and how?
Causes vasodilation via the activation of soluble guanylate cyclase
Activation of soluble guanylate cyclase by NO generates what from what?
Generates cycline Guanine monophosphate (cGMP) from GTP
What breaks down cGMP?
Phosphodiesterases
Activation of eNOS increases what?
Prodcution of NO and therefore vasodilation
What does NO act as an inhibitor to?
- smooth muscle proliferation
- inflammation
- platelet adhesion
Prostacyclin (PGI2) is produced from & by what?
endothelial and smooth muscle cells from arachidonic acid by cyclooxygenase (COX) enzymes.
Is prostacyclin an endothelium-dependent or endothelium-independent
It is a NO-independent vasodilator.
PGI2 induces smooth muscle relaxation by what?
Activating specific cell-surface receptors (IP) that activates adenylyl cyclase and thereby elevating cyclic adenosine monophosphate (cAMP) levels, which induces vasorelaxation.
What can cAMP be degraded by?
Phosphodiesterases
What does prostacyclin activate? What does this do?
Potassium channels and increases efflux of potassium ions, causing membrane hyperpolarization. Membrane hyperpolarization blocks influx of calcium ions required for contraction.
What are natriuretic peptides?
Hormone peptides synthesised by heart, brain and other organs
What stimulates the release of natriuretic peptides?
Atrial and ventricular distension, as well as by neurohumoral stimuli, usually in response to heart failure
What are the three types of natriuretic peptides?
1) Atrial natriuretic peptide (ANP or A-type) produced by the heart,
2) Brain natriuretic peptide (BNP or B-type) produced by the heart,
3) Cellular natriuretic peptide (CNP or C-type), produced in brain, chondrocytes and endothelial cells.
What do natriuretic peptide receptors activate?
Membrane-bound particulate guanylyl cyclase that produce vasorelaxing mediator, cGMP.
Nitric oxide activates only what type of guanylyl cyclases? What does this mean for the action of natriuretic peptide activating membrane-bound guanylyl cyclases?
Cytoplasmic and so both vasorelaxing factors (NO and natriuretic) can act independently from each other.
Why is it important that NO and natriuretic act independently from each other?
This is important because NO and natriuretic peptides can have additive effect on the intracellular pool of cGMP.
Other than vasoconstricting, how does ET-1 function?
- Pro-coagulant
- Activator of vascular smooth muscle proliferation and migration
What enhances the secretion of ET-1?
Physical factors such as shear stress and hypoxia, or chemical stimuli such as thrombin, epinephrine, angiotensin II, growth factors, cytokines and free radicals
What are the two types of receptor and which type do smooth muscle cells produce?
Endothelin-1 receptors, ETA and ETB
Smooth muscle cells produce both
Activation of ETB receptors by ET-1 induces what and how?
vasorelaxation by stimulating endothelial production of NO and prostacyclin. It’s important to remember that the effects of ET-1 depend on the balance between ET-1 receptors expressed on endothelial and smooth muscle cells. While in certain conditions ET-1 may act as a vasorelaxant, increased plasma levels of ET-1 are usually associated with vasoconstriction
What is the difference between ETA and ETB?
ETA: Vasoconstriction & proliferation
ETB: Vasoconstriction
What is angiotensin II?
Peptide hormone that causes vasoconstriction and a subsequent increase in blood pressure. It also acts as a pro-inflammatory factor which increases generation of Reactive Oxygen Species (ROS).
Where are angiotensin II receptors expressed? What does their proportion determine?
receptors 1 and 2 (AT1R and AT2R) are expressed in endothelial cells and their proportion determines vascular effects of angiotensin II
Binding of angiotensin II to AT1R leads to what?
promotes vasoconstriction by enhancing the release of endothelin-1, stimulating production of reactive oxygen species (ROS) that inhibit production of nitric oxide and stimulating production of prostanoid vasoconstrictors such as thromboxane. AT1R also promotes coagulation by increasing release of tissue factor (TF).
Binding of angiotensin II to AT2R leads to what?
AT2R promotes vasodilation by increasing the release of bradykinin which activates eNOS and increases NO levels. AT2R also increases the levels of vasorelaxant prostacyclin.
What is thromboxane?
a vasoconstrictor and pro-coagulant
What is thromboxane produced by?
platelets
What does thromboxane increase the amount of in endothelial cells?
ROS production
What does Thromboxane inhibit in endothelial cells?
NO production
What is maintenance of size-selective sieving property of endothelium crucial for?
Several physiological functions, including normal tissue-fluid homeostasis, angiogenesis, vessel tone, and host defense.
What size molecules is endothelium permeable to?
Endothelium is permeable to molecules ranging from 0.1 nm (sodium ion) to 11.5 nm (immunoglobulin G; IgG) in diameter.
Through what does transcellular transport occur?
vesicular vacuolar organelles
Through what does paracellular transport occur?
tight adherens junctions
How are endothelial cells are anchored and connected to the extracellular matrix (ECM)?
through integrin-based adhesion complexes, namely focal adhesions.
What are Inter-endothelial junctions composed of?
Adherens junctions (AJs) Tight junctions (TJs) Gap Junctions
What do adherens junctions proteins and tight junction proteins associate with?
The actin skeleton at the cytoplasmic side thus providing structural support for the junctions
What does maintenance of endothelial barrier function require?
A balance between contractile and tethering forces within the endothelial monolayer
What are adherens junctions made up of?
Cadherins
What are GAP junctions made up of?
Connexins
What do tethering forces depend on?
The phosphorylation status of junctional proteins and their association with the submembrane actin cytoskeleton
Upon stimulation with factors inducing endothelial permeability, what happens to junctional proteins?
They often become phosphorylated
What does phosphorylation of junctional proteins lead to?
A change in conformational shape and weakens cell-cell binding
Discuss the mode of action by inflammatory mediators such as thrombin, histamine and TNF-a
Rearrangement of the submembrane actin cytoskeleton into stress fibres, contractile actin-myosin filaments that exert a pulling force on the junctions. - thus reducing barrier integrity
What are stress fibres?
contractile bundles of actin and myosin, that act as ‘mini-muscles’ within the cell.
Define vasculogenesis
The differentiation of precursor cells (angioblasts) into endothelial cells and the de novo formation of a vascular network
Define angiogenesis
The growth of blood vessels from the existing vasculature.
What does angiogenesis involve?
The breaking of cell-cell adhesions, basement membrane degradation, cell migration and morphogenesis/coalescence of new vessels
What endothelium-derived mediators regulate agiogenesis
- Nitric oxide
- Growth factors such as vascular endothelial growth factor (VEGF), transforming growth factor-beta (TGF-β), fibroblast growth factor (FGF), platelet-derived endothelial cell growth factor (PDGF)
- Integrins
- Junctional proteins
- Extracellular matrix components
What is angiogenesis triggered by?
Hypoxia
It is important in wound healing, and disease condition e.g. cancer
How does normal, healthy endothelium maintain anti-adhesive properties and blood fluidity?
By producing anti-coagulant factors
When is endothelium triggered to produce pro-coagulants and a reduction of anti-coagulants?
as a result of wounding/inflammation
What are anti-coagulants also known as?
anti-thrombotic substances
Do endothelium cells produce more anti-coagulants or pro-coagulants
anti-coagulants
What is fibrinolysis?
The enzymatic breakdown of fibrin in blood clots
What do prostacyclin, NO, heparan sulphate proteoglycan inhibit?
Platelet aggregation
What does Von Willebrand activate?
Platelet aggregation
What does Thrombomodulin and TF pathway inhibitor inhibit?
Blood coagulation
What does Tissue factor activate?
Blood coagulation
What inhibits and activates fibrinolysis?
- Plasminogen activator inhibitor 1 = inhibits (procoagulant)
- Tissue type plasminogen activator = activates (anticoagulant)
When endothelial cells come in contact with pro-inflammatory agents such as bacterial lipopolysaccharide (LPS) or tumor necrosis factor alpha (TNF-α), what leukocyte-binding receptors do they express?
- selectins (E-and P-selectin),
- intercellular adhesion molecules (ICAM-1 and ICAM-2),
- vascular endothelial adhesion molecule -1 (VCAM-1)
What do leukocyte binding receptors do?
Promote leukocyte adhesion, rolling and transmigration promote leukocyte adhesion, rolling and transmigration
Other than promote leukocyte adhesion, rolling and transmigration, what does P-selectin do?
It encourages adhesion and aggregation of platelets on the surface of endothelium
What produces pro-inflammatory cytokines?
Endothelium cells
What pro-inflammatory cytokines does the endothelium produce?
- Colony stimulating factor-1 (CSF-1)
- Monocyte chemotactic protein-1 (MCP-1)
- Interleukins and
- Chemokine RANTES
What do pro-inflammatory cytokines encourage?
adhesion and transmigration of leukocytes into the underlying smooth muscle cells and extravascular tissues, which helps to combat the disease
What is extravasation?
When neutrophils adhere to capillary wall and then squeeze between the cells (process activated by cytokines following injury)
How is adhesion and rolling possible?
Weak molecular interactions between neutrophils and the endothelium cells form. The neutrophils are slowed down by the formation of weak transient bonds between Sialyl Lewis X (a carbohydrate expressed by neutrophils) and E selectins (an adhesion molecule produced by endothelium cells).
What forces the neutrophils to roll along the wall of the endothelium?
Force of blood flow
During endothelium binding, when does tight binding occur?
When integrins known as LFA1 on the neutrophils undergo a conformational change, causing them to bind firmly with endothelium adhesion molecules called ICAM1 - firm adhesion
What is diapedesis/transmigration and when does it occur?
When the neutrophil is firmly adhered to the capillary wall. It is the process of neutrophils exiting between endothelium cells.
Name the 6 steps of leukocyte adhesion
1) initial capture rolling
2) slow rolling
3) activation/polarisation
4) secondary leukocyte recruitment
5) firm adhesion lateral migration
6) Transmigration (diapedesis)
Can endothelial-1 act as a vasorelaxant?
Yes
