Vascular endothelium

Question 1

What is atherosclerosis?

Answer 1

Chronic inflammatory disease of the arteries due to build up of fatty and fibrous material. It is the underlying condition that causes CHD.

Question 2

Which arteries does atherosclerosis affect?

Answer 2

All, but particularly in those supplying the head, brain and legs.

Question 3

What are some symptoms caused by atherosclerosis?

Answer 3

• angina
• MI
• Stroke
• Peripheral artery disease: arteries supplying limbs narrow, less blood flow

Question 4

What are the three layers of the blood vessels (not in capillaries and venules)

Answer 4

• Tunica Intima: endothelium
• Tunica Media: smooth muscle cells
• Tunica Adventitia: vaso vasorum (blood vessel network supplying the walls of big vessels) and nerves

Question 5

Are all endothelia the same?

Answer 5

No, those in different places have subtle differences e.g. in the kidney

Question 6

How are endothelial cells arranged in vessels and how do they know to arrange like this?
Once endothelial cells have formed are they stable or unstable, and how may this change after injury?

Answer 6

Endothelia form a single layer in vessels
They know that they must divide to form a monolayer and this is called contact inhibition. Once formed the endothelial cells are stable and lots of new ones don’t form. New cells form if new vessels required e.g. after injury. Endothelial cells regulate the functions of blood vessels.

Question 7

What are the main roles of endothelial cells?

Answer 7

• inflammation
• vascular tone and permeability
• angiogenesis
• thrombosis and haemostasis

Question 8

In healthy tissue, what do endothelial cells do?

Answer 8

They maintain an anti-inflammatory, anti-thrombotic state.

Question 9

What happens during inflammation or damage to the endothelium role?

Answer 9

The endothelium flip to produce pro-inflammatory, pro-thrombotic, pro-angiogenic factors.

Question 10

Why is atherosclerosis bad for the blood vessel?

Answer 10

The problem with atherosclerosis is that the endothelium receives a chronic number of stimuli, which translates to cellular signals which keeps the endothelia in an activated state thus stopping it from flipping back to the normal state.

Question 11

When are leukocytes normally recruited?

Answer 11

Recruitment of leukocytes takes place normally during inflammation: leukocytes adhere to the endothelium of post-capillary venules and transmigrate into tissues.

Question 12

What is the process of leukocyte recruitment?

Answer 12

Molecules on the leukocyte allow them to interact with the endothelium but they are normally switched off. Some are not switched off (e.g. selectins) but these don’t have partners on the endothelium to bind to. When inflammation occurs, the endothelium gets activated and it starts to express ligands for the leukocytes. Selectins on the leukocyte interact weakly with the endothelium and make the leukocyte roll. Inside the leukocyte there are signals which activate the integrins (switching them to the high affinity state). The integrins can then strongly bind to the ligands on the endothelium. The leukocyte then binds, adheres and transmigrates.

Question 13

Steps of leukocyte recruitment (simplified)

Answer 13

• capture
• rolling
• slow rolling
• arrest
• adhesion strengthening and spreading
• intravascular crawling
• paracellular and transcellular migration

Question 14

How can leukocytes migrate through the endothelial junctions without them falling apart?

Answer 14

Leukocytes transmigrate by squeezing through endothelial junctions. At the junctions, two endothelial cells are very close to each other and the cell membrane proteins on each cell bind in a homophilic way. This binding of membrane proteins creates a zipper. The junctions can zip and unzip to allow things to go through without the endothelium falling apart.

Question 15

Where does atherosclerosis tend to occur and why?

Answer 15

At branch points in vessels because here the blood flow becomes turbulent. Laminar flow is protective but turbulent flow triggers the activation of inflammatory and thrombotic pathways.

Question 16

What does laminar blood flow promote?

Answer 16

• Nitric oxide production
• Factors that inhibit coagulation, leukocyte adhesion, smooth muscle cell proliferation
• Endothelial survival

Question 17

What does turbulent blood flow promote?

Answer 17

• Coagulation
• Leukocyte adhesion
• Smooth muscle cell proliferation
• Endothelial apoptosis

Question 18

What is angiogenesis?

Answer 18

The formation of new blood vessels by sprouting from pre-existing vessel

Question 19

What controls angiogenesis?

Answer 19

Endothelial cells

Question 20

What triggers angiogenesis?

Answer 20

The requirement of more blood e.g. hypoxia. Chemicals are released that activate the blood vessels and a change is triggered. ‘Tip cell’ controls the new formation.

Question 21

What are the two contrasting effects of angiogenesis on cardiovascular disease?

Answer 21

Angiogenesis plays a detrimental and beneficial role in cardiovascular disease.

• Angiogenesis promotes the growth of atherosclerotic plaques. When you have an advanced plaque and it gets to a size where there is a lot of necrotic debris inside and there is hypoxia - the hypoxia stimulates angiogenesis from the vasa vasorum. These are fragile and more leukocytes will come in and contribute to the growth of the atherosclerotic plaque.
• However, after an MI the heart tissue becomes fibrotic and the patient will develop heart failure. Therapeutic angiogenesis could be used to reoxygenate the myocardium downstream of the occlusion e.g. by using GF or stem cells.

Question 22

Why is it hard to develop anti-atherosclerotic drugs?

Answer 22

The trials are hard and targeting these pathways are dangerous as they are involved in physiological responses. Trying to boost the healthy homeostatic pathways is an option as it is less dangerous

Question 23

What is senescence and why might it be bad?

Answer 23

Senescence is growth arrest that halts the proliferation of ageing and/or damaged cells. Ensures that damaged cells don’t take over and is a protective mechanism against cancer. However, senescent cells can develop a proinflammatory phenotype - may be found in atherosclerotic lesions. As senescent cells have a pro-inflammatory and pro-thrombotic phenotype, they can contribute to atherosclerotic plaque progression.

Question 24

How is red wine related to atherosclerosis?

Answer 24

Rates of mortality in male for CAD in France was low - they drink a lot of red wine. Beneficial effect of red wine in the prevention of atherosclerosis. Molecule involved is called resveratrol.

Question 25

What are the pros and cons of resveratrol?

Answer 25

• It promotes endothelial protective pathways
• Acts as an anti-ageing compound and reduces senescence
• Prevents pro-inflammatory changes in leukocytes
• However red wine is toxic at high doses but beneficial at low (hormetic action)

Question 26

How does an atherosclerotic plaque form (inflammation model) - include details about the endothelium?

Answer 26

At the beginning you have risk factors which activate the endothelium and promotes permeability, leukocyte adhesion and leukocyte migration. Leukocytes which enter the subendothelial layer begin to phagocytose LDLs and form foam cells producing fatty streaks. After a long time, this becomes a large complex plaque with angiogenesis and senescence possible playing a role. Its fate can be either stabilisation by fibrous cap or to rupture.

Question 27

How does blood flow affect epigenetic pathways in endothelial cells?

Answer 27

Three types of mechanisms are involved: DNA methylation, histone modification and RNA (e.g. miRNA).
Stable flow downregulates the expression of DNA methyltransferase which enables the promoters of antiatherogenic genes to remain demethylated, so expressed. Disturbed flow does the opposite causes hypermethylation of genes

Question 28

Why is transmigration of leukocytes bad during atherosclerosis but not physiologically?

Answer 28

Physiologically, transmigration occurs in the post-capillary venules where the leukocyte goes through the endothelial junctions, meets the basement membrane and chews the basement membrane with enzymes allowing it to pass through to the tissue. However if a leukocyte adheres to the inside of a coronary artery or aorta - once it has gone through the endothelium it’s going to find a thick layer which it can not go through and this is how atherosclerosis starts.

Question 29

How does increase in vascular permeability result in the formation of a fatty streak?

Answer 29

Endothelium regulates the flow of substances from the blood to tissues and vice versa. Increased permeability results in the leakage of plasma proteins through endothelial junctions into the subendothelial space. Beneath the endothelium there is a layer of sticky molecules (collagen and proteoglycan). When the endothelium becomes activated,cholesterol goes under the endothelial layer. LDLs are modified as it is a very oxidative environment and are stuck in the subendothelial layer. Macrophages come and phagocytose the LDLs forming foam cells. This is the source of the chronic inflammation.