7: Angiogenesis (30.01.2020) Flashcards
1
Q
What is angiogenesis?
A
- Angiogenesis is the formation of neo-vessels from pre-existing blood vessels
- cascade of events that starts at the endothelial cells, starts in small BVs, starts in response to hypoxia (release of angiogenesis promoting factors e.g. VEGF)
- The angiogenic process is regulated by a wide array of growth factors and signalling pathways
2
Q
Where is angiogenesis seen physiologically and pathologically?
A
Physiology
- Development
- Menstrual cycle
- Wound healing
Pathology
- Cancer
- Chronic inflammatory diseases
- Retinopathies
- Ischemic diseases
- Vascular malformations
Not all BVs are the same, the microenvironements are different.
3
Q
What are the different ways to make a blood vessel>
A
- Vasculogenesis (bone marrow progenitor cell)
- Angiogenesis (sprouting)
- Arteriogenesis (collateral growth; e.g. in occlusion of a large vessel)
=> angiogenesis is not the only way to make new blood vessels
4
Q
Inhibitors of angiogenesis
A
Inhibitors:
- Extracellular Matrix:
- Thrombospondin-1
- Angiostatin
- Endostatin
- Soluble factors:
- sVEGF-R
- IL-10
- IL-12
- TNF-a
- Cell surface receptors:
- avb3
5
Q
Activators of angiogenesis
A
Activators:
- Growth factors:
- VEGF family
- FGF family
- TGF b
- PDGF
- Soluble factors:
- IL-6
- Factor XIII
- TNF-a
- Cell surface receptors:
- avb3
6
Q
Factors for maturation and integrity in angiogenesis
A
Maturation and Integrity: VE-Cadherin (Junctions) Angiopoietin/Tie2 Notch pathway ERG pathway Platelets!!
7
Q
How is angiogenesis regulated?
A
- A large number of molecules can influence Angiogenesis
- Some molecules are essential (i.e. VEGF), other are required for modulation (i.e. VWF)
- Many are best known for other functions (i.e. TNF-a, VWF)
- Some have been reported to have both pro- and anti-angiogenic effects
- Pathways may act in a tissue and stimulus-specific manner
- growth factors play a key role in the regulation of angiogenesis
- many signalling pathways
BALANCE between activators and inhibitors
8
Q
What are the steps of sprouting angiogenesis?
A
- tip/stalk cell selection;
- tip cell navigation and stalk cell proliferation;
- branching coordination;
- stalk elongation, tip cell fusion, and lumen formation;
- perfusion and vessel maturation.
9
Q
How does hypoxia trigger angiogenesis?
A
In the presence of oxygen:
- HIF is bound to pVHL via hydroxyproline (HIF is inhibited by this binding)
- HIF alpha is destroyed
- > HIF is continuously produced and removed
In the absence of oxygen
- pVHL is removed (oxygen take HIF away)
- free HIF alpha
- binds to HIF-beta and DNA
- transcription and translation of:
- VEGF
- PDGF
- EPO
- TGF-alpha
10
Q
HIF and pVHL
A
HIF: hypoxia-inducible transcription factor, controls regulation of gene expression by oxygen
pVHL: Von Hippel–Lindau tumor suppressor gene, controls levels of HIF
=> important in the angiogenesis response to hypoxia!
11
Q
Vascular Endothelial Growth Factor (VEGF) and its receptors
A
- Family of 5 members: VEGF-A, VEGF-B, VEGF-C, VEGF-D, and placental growth factor (PlGF)
- Three tyrosine kinase receptors: VEGF receptor (VEGFR)-1, VEGFR-2, and VEGFR-3; and co-receptors neuropilin (Nrp1 and Nrp2)
- VEGFR-2 is the major mediator of VEGF-dependent angiogenesis, activating signalling pathways that regulate endothelial cell migration, survival, proliferation.
- VEGFR2 and VEGFA are supposed to be the main drivers of angiogenesis.
- when VEGF binds to a cell, this cell beomes a tip cell
12
Q
Tip cell selection
A
- In sprouting angiogenesis, specialised endothelial tip cells lead the outgrowth of blood-vessel sprouts towards gradients of VEGF
- tip cell selection is based on NOTCH signalling between adjacent endothelial cells at the angiogenic front.
- After stimulation with angiogenic factors, the quiescent vessel dilates and an endothelial cell tip cell is selected (DLL4 and JAGGED1) to ensure branch formation.
- Tip-cell formation requires degradation of the basement membrane, pericyte detachment and loosening of endothelial cell junctions.
- Increased permeability permits extravasation of plasma proteins (such as fibrinogen and fibronectin) to deposit a provisional matrix layer, and proteases remodel pre-existing interstitial matrix, all enabling cell migration.
- For simplicity, only the basement membrane between endothelial cells and pericytes is depicted, but in reality, both pericytes and endothelial cells are embedded in this basement membrane.
- Notch R is on the stalk cell and is activated by tip cell in angiogenesis
13
Q
Canonical Notch signalling pathway
A
- notch-R has an ec, tm and ic domain
- binding of ligand (delta/jagged) to notch receptor causes cleavage of ic domain (NICD)
- > translocates to the nucleus and and binds to the TF RBP-J
Summary: activation of the notch R by a ligand causes translocation of the ic-domain(NICD) to the nucleus and binds to TF RBP-J
- Notch receptors are found important in many growth functions, e.g. embryogenesis as well as CNS development, CV development+angiogenesis.
- in angiogenesis the notch-R is on the stalk cell and is activated by the tip cell binding via a ligand (jagged/delta)
14
Q
Dll4
A
- delta like ligand 4
- transmembrane ligand for the notch family of receptors
15
Q
Selection of Tip cells - VEGF and DLL4 signalling
A
- In stable blood vessels, Dll4 and Notch signalling maintain quiescence: the signalling is thought to be balanced in endothelial cells (until presumptive tip cells eventually increase Dll4 expression in response to VEGF signaling)
- VEGF activation increases expression of Dll4
- Dll4 drives Notch signalling, which inhibits expression of VEGFR2 in the adjacent cell.
- Dll4-expressing tip cells acquire a motile, invasive and sprouting phenotype
- Adjaucent cells (Stalk cells) form the base of the emerging sprout, proliferate to support sprout elongation.
16
Q
Sprout outgrowth and guidance
A
- Tip cells navigate in response to guidance signals (such as semaphorins and ephrins) and adhere to the extracellular matrix (mediated by integrins) to migrate.
- Stalk cells behind the tip cell proliferate, elongate and form a lumen, and sprouts fuse to establish a perfused neovessel.
- Proliferating stalk cells attract pericytes and deposit basement membranes to become stabilized.
- Recruited myeloid cells such as tumour-associated macrophages (TAMs) and TIE-2-expressing monocytes (TEMs) can produce pro-angiogenic factors or proteolytically liberate angiogenic growth factors from the ECM.
