1- Angiogenesis and Vasculogenesis

Question 1

Overview of angiogenesis

Answer 1

-­ angiogenesis and vasculogenesis
-­ angiogenic cytokines
-­ role of cytokines in vasculogenesis and
angiogenesis
-­Tumor angiogenesis

Question 2

General concepts of angiogenesis

Answer 2

1- Low turnover but entere active proliferation
2- Linked to pathological processes ( proliferative, tissue repair, ischemia)
3- Regulated by the mechanism that regulate formation of new blood vessels in the embryo
4- A multifactorial phenomenon in steps

Question 3

What is angiogenesis?

Answer 3

The formation of a new vessel from pre-existing vessel

Question 4

When does angiogenesis occur?

Answer 4

Physiological conditions:
1- Embryogenesis and fetal development
2- Endometrial regeneration

Pathological conditions:
1- Wound healing
2- Autoimmune and inflammatory pathologies
3- Tumors

Question 5

Why does agiogenesis occur?

Answer 5

Lack of O2 diffusion that lead to hypoxia which in term lead to apoptosis

Question 6

What are the different types of angiogenesis?

Answer 6

Sprouting angiogenesis and splitting angiogenesis

Question 7

What is Sprouting angiogenesis

Answer 7

The first identified form of angiogenesis.

The process:
1- Biological signals known as angiogenic growth factors activate receptors on endothelial cells present in pre existing veins

2- The activated endothelial cells begin to release enzymes called proteases that degrade the basement membrane in order to allow endothelial cells to escape from the original (parent ) vessel wall

Question 8

What is splitting angiogenesis?

Answer 8

1- Called intussusception
2- The capillary wall extends into the lumen to split single vessels into two.
3- Intussusception is important becasue it is a reorganization of existing cells. It allows a vast increase in the number of capillaries without a corresponding increase in number of endothelial cells. Especially important in embryonic development as there are not enough resources to create a rich microvasculature with new cells every time a new vessel develops.

Question 9

Modern terminology of angiognesis

Answer 9

1- Vasculogenesis
2- Angiogensis
3- Arteriogenesis

Question 10

Define vasculogensis

Answer 10

Formation of vascular structures from circulating or
tissue-­resident endothelial stem cells(angioblasts), which proliferate into
de novo endothelial cells. This form particularly relates to the embryonal
development of the vascular system

Question 11

Define angiogenesis

Answer 11

Formation of thin-­walled endothelium-­lined structures
with /without muscular smooth muscle wall and pericytes (fibrocytes).
This form plays an important role during the adult life span, also as
“repair mechanism” of damaged tissues

Question 12

Arteriogenesis

Answer 12

Formation of medium-­sized blood vessels possessing

tunica media plus adventitia.

Question 13

What is and when does vasculogenesis occur?

Answer 13

Formation of vessels by differentiation of cells from angioblasts

In the embryo: formation of a primitive tubular network
In the adult: differentiation and proliferation of endothelial cells in a non-­
vascularized tissue

Question 14

Organization of Vasculo -, angio- and arterio- genesis

Answer 14

Vasculogenesis: endothelial precursors are assembled to form a network of tiny blood vessels (vascular
plexus) where you can already distinguish between arteries and veins.

Angiogenesis: plexus expands to form an organized network of vessels that branch out gradually.

Arteriogenesis: The vessels are covered by pericytes (PC) and smooth muscle cells (SMC) that allow the
vessels to regulate the blood flow.

Lymphangiogenesis: the lymphatic system develops from veins.

Question 15

How does sprouting occur?

Answer 15

When there is less O2, there are high Hypoxia inducing factor (HIF) allowing the secretion of VEGF.

A growing sprout is composed of :
1- Tip cell - upregulate DII4, VEGFR3, VEGFR2

2- Stalk cells _ upreglate Notch1 and Jagged 1

3- Phalanx cells - upregulate VE-Cadherin , VEGFR1

Question 16

Angiogenic cytokines - VEGF-A

as a protein

Answer 16

1- Family members: VEGF, VEGF-B, VEGF-C, PIGF
2- Dimeric glycoprotein with multiple isoform
3- Targeted mutations in VEGF resulted in defective vasculogenesis and angiogenesis

Question 17

Angiogenic cytokines - VEGF-A

production

Answer 17

1- Expressed at low levels in a variety of adult tissues and at higher levels in a few sites, such as podocytes in the glomerulus and cardiac myocytes

Question 18

Angiogenic cytokines - VEGF-A

inducing agents

Answer 18

1- Hypoxia
2- TGF-B
3- PDGF
4- TGF-A

Question 19

Angiogenic cytokines - VEGF-A

receptors

Answer 19

1- TKR seven ig-like domains in the extracellular portion.
2- One transmembrane domain
3 - On eintracellular tyrosin kinase sequence

4- Expressions:
1. VEGFR1 and R2- Blood vessels
2. VEGFR3: Lymphatic cells
( restricted to endothelial cells)

5- Functions:

1. VEGFR2- mediates proliferation, migration
2. VEGFR-1- does not mediate inhibitory role (decoy receptor for VEGF)
3. Soluble VEGFR-1: isoform of alternative splicing that works as a decoy

Question 20

Angiogenic cytokines - VEGF-A

overall function

Answer 20

1- Promotes angiogenesis
2- Increase vascualr permeability
3- Stimulates endothelial cell migration
4- Stimulates endothelial cell proliferation
5- VEGF-C selectively induces hyperplasia of lymphatic vasculature
6- Up-regulates endothelial expression of plasminogen activator, plasminogen activator inhibitor-1a, tissue factor and interstitial collagenase

Question 21

Angiogenic cytokines - Angiopoietins

Functions

Answer 21

Ang-­1 and Ang-­2 binds to the receptor Tie2

Ang-­1: constitutively expressed by pericytes and vascular smooth muscle cells
promotes vessel integrity, inhibits vascular leakage and suppresses inflammatory gene expression

Ang-­2: stored in Weibel–Palade bodies and rapidly secreted and induced upon stimulation
works as an antagonist of Tie2 inhibiting its protective signalling, blocks Tie 2 protective signalling

Question 22

Role of cytokines in vasculogenesis

Answer 22

1) VEGF binds to VEGF receptor-­R2 present on mesangioblast and induces its endothelial differentiation and its proliferation. Knock-­out mice for
VEGFR2 die on Day 8 of embryonic life and have no
endothelial cells

2) The capillary tube formation occurs when VEGF binds to VEGF-­R1. Knock-­out mice for VEGFR1 dies on Day 8 of embryonic life due to the lack of vessels.
3) Ang-­1 binds to Tie2 and allows the recruitment of pericytes and smooth muscle cells that maintain the stability of the vessel.In the Tie2 knockout mouse vessels are formed, but they break quickly. Death at 9-­10 days of embryonic life.
4) Ang1 and TGFb: stabilize the nascent vessels
5) Ang2: destabilizes blood vessels (angiogenesis or regression)

Question 23

Angiogenesis in pathology

Answer 23

The balance between pro-­and anti-­angiogenic factors is
altered in favor of angiogenesis in various diseases: it is
called angiogenic switch

Tumors
Psoriasis
Inflammatory diseases (arthritis ...)
diabetic retinopathy
Hemangiomas

Question 24

how does angiogenesis control tumor growth

Answer 24

• Tumor can stay in a ‘dormant’ state for
long periods.
• The angiogenic switch allows for growth of
the tumor to occur.
• Increased angiogenesis correlates with
worse prognosis.

Question 25

Explain the growth of tumor vessels

Answer 25

1- Sprouting
2- Intussusceptive growth
3- Incorporation of BM-deerived precursors
4- Cooption of existing vessels

Question 26

VSP inhibitors and TKIs

52

Answer 26

1- Bevacizumab (Anti-VEGF)
2- Afliberecept (VEGF Trap)
3- Ramucirumab (Anti-VEGFR2)

and TKIs