Angiogenesis lecture 11 Flashcards
What is the difference between angiogenesis and vasculogenesis?
Angiogenesis is the formation of blood vessels from other blood vessels,
vasculogenesis is the formation of new blood vessels from mesodermal derived angioblasts
List the roles of angiogenesis
During development
Physiological processes- wound healing, menstrual cycle, adaption to increased muscle activity
Pathological processes- cancer (cancer cells form a blood supply), inflammatory, diabetic retinopathy
Describe the process of vasculogenesis
Endothelial progenitors (angioblasts) differentiate from mesodermal cells
Angioblasts coalesce to form first embryonic vessels- dorsal aorta and cardinal vein (formed by forming a column which is hollow allowing blood to move)
Angioblasts form blood islands which fuse and remodel to form a primitive capillary plexus (netwrok of vessels to delveer O2 and nutrients effciently)
Describe angiogenesis during development
Angiogenic remodelling of dorsal aorta, cardinal vein and vascular plexi give rise to arteries, veins and capillaries
Angiogenic sprouting of new vessels
Pericytes are recruited to stabilise capillaries
Lymphatic endothelial cells sprout from veins and form they lymphatic system via lymphangiogenesis
O2 and the antomical location drives angiognesis
ONce you get around 50um away from a vessel, O2 levels are nearly 0. O2, glucose and ATP levels will drop the furher you move away from vessel due to diffusion. As you move away O2 levels drop so you have to use glycolysis to form O2 which causes a large increase in lactic acid.
What is the main cause of physiological angiogenesis?
Hypoxia- Hypoxia Inducible Factor (HIF)-1 is a transcription factor and a master regulator of oxygen homeostasis
Consists of alpha and beta subunits
Degradation of HIF-1alpha is regulated in response oxygen levels
In high levels a proline hydroxylase hydroxylases proline residues on the alpha subunit so it is then recognised and bind by ubiquitin ligase which polyubiquitates it to be degraded
If there isn’t enough oxygen to drive the proline hydroxylation the alpha subunit associates with the beta subunit and activates its target genes eg. VEGF - pro-angiogenic factors (Angiogenesis occurs when the effects of proangiogenic activators are greater than the inhibitors)
What is the function of VEGF in endothelial cells?
VEGF-A is the potent driver of angiogenesis
VEGFR signals via activation of VEGFR-2 a receptor tyrosine kinase
Dimerisation of VEGFR-2 activates signalling pathways which affects- proliferation, survival, migration, vascular permeability (regulation of adheren junctions)
Summarise the modes of angiogenesis
New vessels can go by Sprouting or splitting (intussusception)
Induced by shear stress
Intussusception involved the formation of a pillar which elongates splitting the vessel in two- rapid increase in capillarity and requires minimal endothelial proliferation
Tissue stretch- Sprouting involves the formation of a new capillary off another
Describe the stages of sprouting
1) VEGF regulated VE-cadherin and loosens endothelial junctions
2) Increased permeability allows the deposition of plasma proteins to make a provisional matrix
3) Endothelial cells secrete proteases to remodel existing interstitial matrix
4) Selection of tip cell to guide the newly forming sprout- filopodia sense the extracellular environment and secrete VEGF .
5) Pericyte detachment mediated by Angiopoeitin
6) Tip cells navigate in response to guidance signals and adhere to the extracellular matrix (mediated by integrins) to migrate
7) Notch signalling between the tip (delta-like 4/DL4) and stalk cells (notch) maintains their different specification. Cells express DL4 which signals by notch to neighbouring cells and downregulates tip cels forming.
8) Stalk cells behind the top proliferate and extend the sprout- VEGFR-2 is down regulated
9) Formation of the lumen in the newly developing sprout
10) Stalk attracts pericytes to stabilise the newly formed vessels
11) Sprouts from adjacent vessels grow towards each other
12) Angiopoeitin 1 signalling between endothelial cells and pericytes maintain quiescence of the vessel
13) Formation of tight junctions, basement membrane deposition, pericytes maturation
TISSUE STRECTH induces angiogenesis (in muscle contraction)
Summarise pathological angiogenesis
Tumour growth and hyperplasia in inflammatory diseases increase the distance of cells from vessels and chronic hypoxia drives VEGF production plus inflammatory cytokines gives rise to a pro-angiogenic environment
Vessel formation is abnormal leading to leaky and poorly perfused vessels and persistent hypoxia
How would you modulate angiogenesis via VEGF? (take out VEGF)
Inhibit signalling using a monoclonal antibody eg. Bevacizumab, used in cancers and diabetic retinopathy. Mops up excess VEGF
Use a soluble receptor- VEGF trap - prevent VEGF binding to other recptors
Small molecule inhibitors eg. Sunitinib, sorafenib used in cancer treatment (stops tyrosine kinase domain from functioning)
Antibody blocking binding if VEGF to the receptor eg. IMC 1C11
Assays to assess angiogenic sprouting
Promoting angiogenesis in ischaemic disease - promting angiogensis has the potential ro revascularise ichaemic diease - want to grow new vessels
Can asses angiogenesis by:
- THe spheroid assay - suspend cells which settle and form a ball which can be embedded in collagen which then will sprout. Use colour labeling to look at them and can manipulate gene expression.
- The co-culture assay - firboplast produce VEGF, FGF-2 added to co-cultute caussinf growth of blood vessle s and when you inhibit it, it doesnt form vessels.
Can use zebrafish to look at funciton of genes in angiogenesis as they are transparent
