Vascular Endothelium Flashcards
Describe the extent of the vascular system
one of the largest and the most extensive networks in the body
What percentage of endothelial cells are in the microvascular system?
98%
What is the basic structure of blood vessels?
Tunica adventitia
Tunica media
Tunica intimia
What are vasa vasorum?
Tiny vessels that feed larger vessels
What is the structure of capillaries and venules?
only by endothelium, supported by some mural cells (pericytes) and a basement membrane
What happens in capillaries?
exchanges of nutrients and oxygen between blood and tissues occur
What is the function of the microvascuar endothelium?
promotes tissue homeostasis
How does the microvascular endothelium promote tissue homeostasis?
Microvascular endothelium is the source of angiocrine factors required for the maintenance of tissue homeostasis and organ regeneration
Dysfunctional endothelium contributes to disease more than any other “organ”: give examples of some diseases
Ischemia
Chronic inflammatory diseases
Cancer
Diabetes
How is the microvascular endothelium different between systems?
Endothelial cells and microvasculature have organotypic (tissue-specific) properties and expression profiles
What is the basic concept of the endothelial cells?
Flat
Large surface area
From one cell deep monolayer
Acts as a vital barrier separating blood from tissues
What is contact inhibition?
Process that takes place when the junction between to cells come together
The establishment of the junction signals the cell to stop growing
What is the lifespan of endothelial cells?
In vivo, endothelial cells live a long life and have a low proliferation rate (unless new vessels are required: angiogenesis)
What are the multiple functions of blood vessels?
Tissue homeostasis and regeneration Vascular tone Angiogenesis Permeability Inflammation Haemostasis & Thrombosis
All controlled by endothelium
What are the products of endothelial cells that regulate thrombosis and haemostasis?
Antithrombic factors
Procoagulant factors
What pathways are activated when a trigger is present?
Pro-inflammatory
Pro-thrombic
Pro-angiogenic
What triggers can activate endothelium?
Smoking Viruses Mechanical stress Inflammation High blood pressure OxLDL High glucose
How can the endothelium respond to triggers?
Thrombosis
Senescence
Permeability
Leukocyte recruitment
What are the products of endothelial cells that regulate angiogenesis?
Matrix Products
Growth factors
What are the products of endothelial cells that regulate inflammation?
Adhesion molecules
Inflammatory mediators
What are the products of endothelial cells that regulate vascular tone permeability?
Vasodilator factors
Vasoconstricting factors
What is the pathogenesis of atherosclerosis?
Endothelial dysfunction - activation causes increase in permeability and upregulation of the systems that promote the adhesion of leukocytes
Leukocytes migrate and accumulate in the sub-endothelial space
Phagocytosis of the lipids in the sub-endothelial space leads to
Fatty-streak formation
Formation of necrotic core
Angiogenesis
What stimuli cause atherosclerosis?
Hypercholesterolaemia Diabetes Mellitus Hypertension Sex hormonal imbalance Ageing Oxidative stress Proinflammatory cytokines Infectious agents Environmental toxins Hemodynamic factors
What are the 4 mechanisms that contribute to the formation of atherosclerotic plaques?
Leukocyte recruitment
Permeability
Shear stress
Angiogenesis
What are the steps of the leukocyte adhesion cascade?
Rolling Activation Arrest Adhesion Transmigration
What is the role of the endothelium in the adhesion cascade?
Endothelium expresses molecules that supports the processes of the adhesion cascade
Where does recruitment into tissues normally take place?
During inflammation
In post-capillary venules
Where does recruitment of leukocytes occur in atherosclerosis?
leukocytes adhere to activated endothelium of large arteries and get stuck in the subendothelial space
Monocytes migrate into the subendothelial space, differentiate into macrophages and become foam cells
What is the function of the endothelium relating to permeability?
The endothelium regulates the flux of fluids and molecules from blood to tissues and vice versa
What is the consequence of increased permeability?
Increased permeability results in leakage of plasma proteins through the junctions into the subendothelial space
Where do atherosclerotic plaques occur preferentially?
Bifurcations and curvatures of the vascular tree
Why do they occur preferentially ?
The flow patterns and hemodynamic forces are not uniform in the vascular system
How do straight and curved parts of vessels affect flow?
In straight parts of the arterial tree, blood flow is laminar and wall shear stress is high and directional
In branches and curvatures, blood flow is disturbed with nonuniform and irregular distribution of low wall shear stress.
What does laminar flow promote?
It is protective
anti-thrombotic, anti-inflammatory factors
endothelial survival
Inhibition of SMC proliferation
Nitric oxide (NO) production
What does disturbed flow promote?
Thrombosis, inflammation (leukocyte adhesion)
endothelial apoptosis
SMC proliferation
Loss of Nitric oxide (NO) production
What are the protective effects of nitric oxide on the cardiovascular system?
Dilates blood vessels Reduces platelet activation Inhibits monocyte adhesion Reduces proliferation of SMC in the vessel wall Reduces releases of superoxide radicals Reduces oxidation of LDL cholesterol
What is angiogenesis?
The formation of new vessels by sprouting from existing vessels
What triggers angiogenesis?
Hypoxia
What are the steps of angiogenesis?
Endothelial cell recepetor binding
EC activation
EC proliferation
Driectional migration
ECM remodelling
Tube formation
Loop formation
Vascular stabilisation
What does angiogenesis play a key role in?
Embryonic development
Wound healing
Menstrual cycles
What are the good and bad outcome of angiogenesis in cardiovascular disease?
Bad- angiogenesis promotes plaque growth
Good- therapeutic angiogenesis prevents damage post-ischaemia
What are frequent in COVID patients?
Venous and arterial thrombosis
What does loss of normal antithrombotic and anti-inflammatory functions cause?
thrombosis with associated inflammation: thromboinflammation
When do you get thromboinflammation?
Occurs in many disorders, including sepsis, ischemia-reperfusion injury etc.
What does COVID trigger?
Cytokine storm
Endothelical activation
Procoagulant switch
What are the 2 possible mechanism that covid can cause endothelial damage?
Cytokine storm
Directly
Does SARS-CoV2 actually bind endothelial cells?
Lack of evidence
ACE2 expressed on epithelial not endothelium
What research is ongoing?
Research into drugs that prevent or reverse endothelial dysfunction are likely to benefit many diseases including COVID
What are the modifiable risk factors for atherscelrosis?
Smoking Lipids intake BP Diabetes Obesity Sedentary lifestyle
Multiple of the above increase risk greatly
What are the non-modifiable risk factors for atherosclerosis
Age
Sex
Genetic background
What are the recent epidemiological changes relating to atherosclerosis?
Reduced hyperlipidaema
Reduces hypertension
Increased obesity and diabetes
Changing pathology of coronary thrombosis
How do LDLs contribute to atherosclerosis?
Low density lipoproteins (LDL) deposit in the subintimal space and binds to matrix proteoglycans
What are the stages of atherosclerosis progression?
Lesion prone location- adaptive thickening
Type II lesion - macrophage foam cells
Type III prearthroma - small pools of extracellular lipid
Type IV atheroma - core of extracellular lipid
Type V fibroatheroma - Fibrous thickening
Type VI complicated lesion -Fissure and hematoma and thrombosis
Type VI lesion
What is the natural progression of atherosclerosis?
Normal
Intermediate lesions
Advanced lesions
Complications
What are the main complications of atherosclerosis?
stenosis, plaque rupture
What are the main cell types involved?
Vascular endothelial cells Monocyte macrophages Vascular smooth muscle cells Platelets lymphocytes
What are the available clinical interventions for atherosclerosis?
Secondary prevention
Catheter based interventions
Revascularisation surgery
Treatment of heart failure
How was the role of inflammation in atherosclerosis discovered?
Patients injected with antibodies to Interleukin 1
Had fewer cardiovascular events
What is the death zone?
Lipid necrotic core
Dead/dying cells that release cholesterol
Cholesterol crystallizes
How can WBCs cause harm?
White blood cells can injure host tissue if they are activated excessively or inappropriately!
What are the two main classes of macrophage?
Inflammatory
Resident
What is the function of inflammatory macrophages?
Adapted to kill microorganisms (germs)
What are the functions of resident macrophages?
Normally homeostatic - suppress inflammatory activity
Alveolar resident macrophages - surfactant lipid homeostasis
Osteoclasts - calcium and phosphate homeostasis
Spleen - iron homeostasis
What are the characteristics of LDLs?
Bad’ cholesterol - Synthesised in liver.
Carries cholesterol from liver to rest of the body including arteries.
But is required in small amounts
What are the characteristics of HDLs?
Good’ cholesterol
Carries cholesterol from ‘peripheral tissues’ including arteries back to liver (=“reverse cholesterol transport”)
What are oxLDLs and modified LDLs?
Oxidation due to action of free radicals on LDL
Not one single substance
Families of highly inflammatory and toxic forms of LDL found in vessel walls
How is subendothelial trapped LDL modified?
LDLs leak through the endothelial barrier by uncertain mechanisms.
LDL is trapped by binding to sticky matrix carbohydrates (proteoglycans) in the sub-endothelial layer and becomes susceptible to modification
LDL becomes oxidatively modified by free radicals. Oxidised LDL is phagocytosed by macrophages and stimulates chronic inflammation!
What are the features of familial hyperlipidemia?
Autosomal genetic disease
Massively elevated cholesterol (20mmol/L).
Failure to clear LDL from blood.
Xanthomas and early atherosclerosis; if untreated fatal myocardial infarction before age 20.
What did the discovery of LDL receptor lead to?
Cholesterol synthesis is also negatively regulated by cellular cholesterol. Led to the discovery of HMG-CoA reductase inhibitors (= “statins”) for lowering plasma cholesterol.
What is the function of Macrophage scavenger receptor A?
Known as CD204
Binds to oxidised LDL
Binds to Gram-positive bacteria like Staphylococci & Streptococci
Binds to dead cells
What is the function of Macrophage scavenger receptor B?
Known as CD36
Binds to oxidised LDL
Binds to malaria parasites
Binds to dead cells
What are the functions of activated macrophages?
Generate free radicals that further oxidise lipoproteins
Phagocytose modified lipoproteins, & become foam cells
Express cytokine mediators that recruit monocytes
Express chemo-attractants & growth factors for VSMC
Express Proteinases that degrade tissue
How do macrophages generate free radicals?
Macrophages have oxidative enzymes that can modify native LDL.
a) NADPH Oxidase, for example superoxide O2-.
b) Myeloperoxidase, for example, HOCl hypochlorous acid (bleach) from ROS + Cl-, HONOO Peroxynitrite.
How do macrophages phagocytose?
Macrophages accumulate modified LDLs to become enlarged foam cells.
How do macrophages express cytokine mediators?
Plaque macrophages express inflammatory factors that are involved in monocyte recruitment.
Positive feedback loop / vicious cycle leading to self-perpetuating inflammation.
What are cytokines?
Cytokines – protein immune hormones that activate endothelial cell adhesion molecules
What are chemokines?
Chemokines - small proteins chemoattractant to monocytes
What cytokines do macrophages produce?
Interleukin-1 upregulates vascular cell adhesion molecule 1 VCAM-1
VCAM-1 mediates tight monocyte binding
Atherosclerosis is reduced in mice without IL-1 or VCAM-1
What chemokines do macrophages produce?
Monocyte chemotactic protein-1 (MCP-1)
MCP-1 binds to a monocyte G-protein coupled receptor CCR2.
Atherosclerosis is reduced in MCP-1 or CCR2 deficient mice.
How do macrophages promote wound healing?
Macrophages release complementary protein growth factors that recruit VSMC and stimulate them to proliferate and deposit extracellular matrix
What chemo-attractants and growth factors do macrophages express?
Platelet derived growth factor:
Vascular smooth muscle cell chemotaxis
Vascular smooth muscle cell survival
Vascular smooth muscle cell division
Transforming growth factor beta:
Increased collagen synthesis
Matrix deposition
How do macrophages express proteinases that degrade tissue?
Metalloproteinases (=MMPs) Family of ~28 homologous enzymes. Activate each other by proteolysis. Degrade collagen. Catalytic mechanism based on Zn.
What are the characteristics of vulnerable and stable plaques?
Large soft eccentric lipid-rich necrotic core
Increased VSMC apoptosis
Reduced VSMC & collagen content
Thin fibrous cap
Infiltrate of activated
macrophages expressing MMPs
How can macrophages go through apoptosis?
OxLDL derived metabolites are toxic eg 7-keto-cholesterol
Macrophage foam cells have protective systems that maintain survival in face of toxic lipid loading.
Once overwhelmed, macrophages die via apoptosis.
What do dead macrophages result in?
Release macrophage tissue factors and toxic lipids into the ‘central death zone’ called lipid necrotic core.
Thrombogenic and toxic material accumulates, walled off, until plaque rupture causes it to meet blood.
What is Nucleasr Factor Kappa B (NFkB)?
Transcription Factor
Master regulator of inflammation
How does NFkB work?
Switches on numerous inflammatory genes:
Matrix metalloproteinases
Inducible nitric oxide synthase
Interleukin-1
What activates NFkB?
Activated by numerous inflammatory stimuli:
Scavenger receptors
Toll-like receptors
Cytokine receptors e.g. IL-1
