Angiogenesis

Question 1

What are the physiological occasions for angiogenesis?

Answer 1

• embryonic development
• menstrual cycle
• wound healing

Question 2

Pathological needs for angiogenesis?

Answer 2

Many diseases have been shown to be dependent on angiogenesis, or linked by some sort of angiogenesis dysfunction (e.g. insufficient or excessive angiogenesis).

Question 3

How is a blood vessels made (what are the different ways)?

Answer 3

Vasculogenesis – bone marrow progenitor cell (during development). It involves progenitors that form a blood vessel from scratch)

Angiogenesis – sprouting (this is the most common in adults and involved in disease processes). Vessels sprout from a pre-existing blood vessels

Arteriogenesis – collateral growth (mechanism through which collaterals are formed). Collaterals may bypass a blockage.

Question 4

What is the model of sprouting angiogenesis?

Answer 4

• Angiogenesis affects small blood vessels, with endothelial cells and matrix
• There is a pro-angiogenic stimulus, which triggers the activation of specific selected endothelial cells
• The endothelial cell must drive the formation of the blood vessel. It undergoes a change
• The cytoskeleton changes polarity
• This allows it to sense the outside world (senses a stimulus that allows the direction of blood vessel formation)
• At the same time, the cell has to be in touch with other cells
• The cell needs to communicate with other nearby endothelial cells, in order to instruct them to divide, to form vessels
• Cells begin to chew up the matrix, and eventually they fuse with a sprout coming from elsewhere

Question 5

Which cells are important in angiogenesis?

Answer 5

endothelial tip cells

Question 6

What happens during angiogenesis?

Answer 6

• Growth factors are released that activate endothelial cells in the pre-existing capillaries
• The endothelial cells undergo a conformational change
• They go from being part of a very organised monolayer, to sending out filopodia
• Endothelial cells begin to migrate towards the growth factors
• To allow the endothelial cell to do this, the cytoskeleton of the tip cell must be modified and it needs to control the interaction with neighbouring cells at cell-cell junctions
• The tip cells will keep on moving until they find another tip cell, with which they will fuse
• The tip cells themselves do not divide
• They require their neighbouring cells to divide behind them to push the tip cells towards the GF
• Eventually, the tip cell will meet another tip cell and it will fuse and stabilise

Question 7

When may blood vessels be needed?

Answer 7

Usually hypoxia

Question 8

How is angiogenesis regulated?

Answer 8

• When you form a new vessel, you need to destabilise the pre-existing vessel and then re-stabilise it
• There are activators and inhibitors of angiogenesis
• A balance of these 2 groups regulates angiogenesis

Question 9

Give an example of a regulator of angiogenesis

Answer 9

VEGF - loss of one allele is incompatible with life (other regulators are less important and loss of them would result in abnormal vasculature)

Question 10

How does hypoxia trigger angiogenesis?

Answer 10

• HIF is a growth factor that controls regulation of gene expression by oxygen
• When oxygen is plentiful, HIF transcription factor is bound by a protein: pVHL (Von Hippel-Lindau) – a tumour suppressor gene
• When bound to HIF, pVHL induces ubiquitination (inactivation by the addition of ubiquitin) and degrades HIF
• The moment oxygen is not plentiful, pVHL no longer binds to HIF -> HIF is not degraded and enters the nucleus to bind HIF-beta -> drives transcription of genes that promote angiogenesis, such as VEGF

Question 11

What is VEGF?

Answer 11

The best-known pro-angiogenic growth facto

Question 12

What are the members of the VEGF family?

Answer 12

5 members of the VEGF family: VEGF-A, VEGF-B, VEGF-C, VEGF-D, PIGF (placental GF)

Question 13

What are the receptors for VEGF?

Answer 13

3 tyrosine kinase receptors for VEGF: VEGFR-1, VEGFR-2, VEGFR-3

(They combine in different ways to form dimers)

2 co-receptors for VEGF: Neuropilin-1 (Nrp1) and Neuropilin-2 (Nrp2)

Question 14

Which receptor is the main mediator of VEGF dependent angiogenesis?
What does activation of it do?

Answer 14

VEGFR-2

It activates signalling pathways that regulate endothelial cell migration, survival and proliferation

Question 15

What are endothelial tip cells?

Answer 15

• In sprouting angiogenesis, specialised endothelial tip cells lead the outgrowth of blood-vessel sprouts towards gradients of VEGF
• Once a tip cell has been selected, it seems to control the behaviour of the cells around it via cell-cell communication

Question 16

How are tip cells selected?

Answer 16

There is a pathway called Notch that is crucial for the selection of tip cells. Tip cell selection is based on notch signalling between adjacent endothelial cells at the angiogenic front

Question 17

….

Answer 17

• At the very beginning, you have a quiescent vessel
• In angiogenesis, the cells exit quiescence and proliferate rapidly
• VEGF is one of the most important triggers for this transformation from quiescent cells -> tip cells
• A lot of other things must happen simultaneously – one cell is a tip cell, the other cells must divide

Question 18

…..

Answer 18

Endothelial cells are some of the slowest dividing cells in the body

Question 19

What is notch signalling?

Answer 19

• It is a signalling pathway that is not specific to endothelial cells (it is essential for development)
• Notch receptors and ligands are membrane-bound proteins that associate through their extracellular domains
• Binding of the notch ligand to the notch receptor activates it by cleaving the intracellular domain (NICD)
• NICD translocates to the nucleus and binds to the transcription factor RBP-J
• This is a way of going from cell-cell communication, to telling the nucleus what to do

Question 20

What happens when a tip cell is selected and why?

Answer 20

• When a tip cell is chosen, it begins to express notch ligand which binds to the stalk cells’ notch receptors
• It tells them that ‘I am the tip cell, you are the stalk cells’
• The stalk cells then begin to divide and push the tip cell towards the growth factor

Question 21

What is the notch ligand also known as?

Answer 21

Delta-like ligand 4 (Dll4)