Endothelium

Question 1

Where is the endothelium found?

Answer 1

A thin monolayer of endothelial cells around found in the inner layer of blood vessels

Blood vessel composition:
Endothelium
Smooth muscle cell
Elastic/connective tissue

Question 2

What does the endothelium do?

Answer 2

Produces molecules that are used in dependent vasodilation, antithrombotic, anti-inflammatory, anticoagulant/prothrombotic and antihypertrophic
Example - nitrix oxide

It works synergistically with the smooth muscle cells - to thelp maintain arterial pressure

Question 3

Describe the endothelium’s function in health?

Answer 3

Nitric oxide is synthesised by the endothelium

It dilates blood vessels
It reduces platelet stickiness
Reduces monocyte stickiness - preventing the formation of plaque
Reduces multiplication (out of control) of smooth muscle cells of the artery wall
Reduces release of superoxide radicals
Reduces oxidation of LDL cholesterol (major component of plaque)

Question 4

Describe the endothelial diversity?

Answer 4

Different endothelial origins or environments may influence vascular physiology control i.e. in different vessels or regions of the body

The endothelial cell is an input-output device
Responds to different surroundings, environments and changes
Responds to both biochemical and mechanical changes

Question 5

What are the different phenotypes of the endothelium within the vessels?

Answer 5

Capillary:
Continuous not perforated - skin, lung, heart
Continuous perforated - endocrine glands, GI mucosa, glomerulus
Discontinuous/sinusoidal - liver

Post-capillary venule
Lumen - small intercellular cleft
During inflammation - widening of the intercellular cleft, allowing fluids and solutes to enter

Question 6

What is involved in the input-output activity of the endothelium?

Answer 6

Cell adhesion
Response to growth factors
Interaction with migrating cells
Balance between dormancy, proliferation and death

Depends on the input to the role it plays

Question 7

What are some key physiological features the endothelium exhibits?

Answer 7

De novo blood vessel network formation (vasculogenesis)
New blood vessel sprouting (angiogenesis)
Vascular homeostasis e.g. blood pressure control
Blood clotting
Disease related functions and dysfunction

Question 8

Describe endothalial cell adhesion?

Answer 8

Endothelial cells have key adhesive functions:
Provision of an intercellular barrier
Regulation movement of molecules
Regulation of cell movement
Endothelial barrier function regulates angiogenesis and vasculogenesis

Question 9

Give an example of endothelial adhesion?

Answer 9

Interaction between endothelial cell and leukocytes are essential for inflammatory processes
Leukocytes become activated and transmigrate between the endothelial cells into surrounding tissue
Here in the surrounding tissue the inflammatory response can begin

Question 10

Describe the balance the endothelium is trying to regulate?

Answer 10

Dilation, growth inhibition, antithrombosis, anti-inflammation

Constriction, growth promotion, pro-thrombosis, pro-inflammation

At any one time they are trying to regulate

Question 11

Describe angiogenesis?

Answer 11

The endothelium cells form new blood vessels that sprout from it

Question 12

How is angiogenesis linked to cancer?

Answer 12

Innervation of solid tumour with leaky blood vessels
Solid tumour growth depends on angiogenesis
Spread of tumour cells via the vascular network to other sites (metastasis)

Question 13

What stimulates angiogenesis?

Answer 13

Hypoxia
Low (<1%) oxygen = hypoxia linked to sold tumour growth and spread
Hypoxia-regulated synthesis of pro-angiogenic factors

Hypoxia stimulates pro-angiogenic VEGF-A synthesis
VEGF-A secretion stimulates angiogenesis
VEGF-A is dependent on HIF-1 activity

VEGF-A is secreted and binds to cognate receptors on the endothelium
Blocking such interactions can be used to treat cancer e.g. Avastin and Sutent

Question 14

Describe HIF-1 proteins

Answer 14

HIF protein complexes regulate gene transcription e.g. VEGFA locus
HIF-1a subunits undergo, ubiquitination for degradation in high oxygen (>1% O2)
HIF-1b subunit is stable
HIF subunits have gene expression regulatory properties

HIF’s are used in oxygen regulation of multiple vascular responses and disease states

Question 15

What describes the surfaces of the endothelial cell?

Answer 15

Endothelial Glycocalyx

This is a carbohydrate-rich layer lining the vascular endothelium
It is connected to the endothelium through several “backbone” molecules:
Cell surface glycoproteins
Glycolipids
Proteoglycans

Responds to different substances in blood and other fluids

Question 16

What are some receptors in the endothelial glycocalyx?

Answer 16

Vascular endothelial growth factor (VEGF) receptors
They are a type of tyrosine receptor kinases (TRK)
There are 3 receptors 1-3

There are 5 VEGF’s A-E
Multiple splice variants encoded by each gene - they can bind to various receptors for different cellular responses
VEGF-A is the most important as it is closely associated with tumour growth and progression

Question 17

What is involved in angiogenesis regulation?

Answer 17

Blood vessel sprouts express Dll4 (ligand for Notch)
Stalk cells express unique cell surface receptors - Notch and VEGFR2

VEGFR2 signalling positively regulates angiogenesis
Notch signalling negatively regulates angiogenesis

Question 18

Describe the endothelial adhesion interaction with leukocytes?

Answer 18

Leukocyte adhesion-cascade:
This uses many cells and molecules - the movement of white blood cells

Capture of the leukocyte on the endothelial cell
Selectins- bind sialyl-LewisX sugars, found on eg PSLG-, low affinity, transient in nature
They signal via phosphoinositide-3-kinase(gamma) (PIK(gamma)) resulting in slow rolling along the endothelial layer

Activation through many molecules
Arrest - leukocyte stops rolling (mediated by integrins)

Adhesion is then strengthened by spreading
Then intravascular crawling where paracellular and transcellular transmigration takes place
(Leukocyte squeezes through the endothlial layer) - moving into the extra cellular matrix

Question 19

Describe atherosclerosis and the role of the endothelium?

Answer 19

Atherosclerosis:
Plaques or lesions
Fat-filled streaks or lesions
Major artery blockage by blood clots
Causes heart attacks or strokes

Monocytes are a type of leukocyte which bind to the endothelium
Monocytes differentiate into macrophages which bind modified LDLs
Fat and lipid-filled macrophages become foam cells that die
Atherosclerotic plaque lesion starts to appear in the blood vessel wall

This leads to atherosclerotic plaque rupture

Question 20

Describe endothelial cell differentiation?

Answer 20

Endothelial cells are derived from stem cells
Differentiation pathway during normal development
Can be derived from circulating stem cells during adult life
Provides potential in therapy
They have different markers that we can follow

Question 21

Describe endothalial stem cell therapy?

Answer 21

Isolation of circulating Endothelial Progenitor Cells (EPCs)
Differentiation in vitro to ECs (in the test tube or lab)
Re-implantation of differentiated cells into damaged tissue in vivo e.g. damaged heart or blood vessel tissues

Question 22

What are endothelial derived tumours?

Answer 22

Kaposi’s Sarcoma Herpes Virus (KSHV)
Linked to HIV infection
Infects endothelial cells
Promotes endothelial growth and immortalisation
Forms metastatic tumours

KSHV encoded gene mediates endothelial cell transformation
Viral GPCR
Blocks cell death
Elevates signalling and expression of pro-survival/proliferative growth factors