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Major regulator of vegf-dependent sprouting nagiogenesis ? what does it regulate? evidence

VEGFR-2 receptor. activates a variety of downstream signalling pathways that regulate EC migraiton, survival, proliferation, vessel formation.

in mice, loss of even a single VEGF allele results in emryonic leatlity at 11 days due to vascular defects.


tip cells do what?

lead the outghrowth of blood-vessel sprouts towads gradients of VEGF, principally VEGF-A.

the tip cell uses fillopodia to scan the environment for attractive and repulsive cues. they adhere to ECM *mediated by integrins) to migrate


what does the formation of tip cell require?

degradation of the basement membrane (MMPs)
pericyte detachment (ANG-2)
loosening of endothelial cell junctions (VE-cadherin)

VEGF mediates increased permeability, vasodilation and extravasation of plasma proteins, which allows depositing a provisional matrix layer and proteases remodel pre-existing ECM, all enabling cell migration.


Notch signalling pathway

notch cignalling beginds with receptor-ligand the interaction between neigbouring cells. this triggers a series of proteolytic cleavages of the notch receptor. the final one release the active Notch intracellular domain (NICD).

NICD is translocated to the nucleaus where it interacts directly with the transcription factors CSL. this complex becomes an activating complex for the target genes, amongst which is also gene coding for Dll4, and it is downregulated upon Notch stimulation.


Stalk cells are..

also activated endothelial cells.
they are behind the tip cell,
they prolifete, elongate and for a lumen, and sprouts fuse to establish a perfused neovessel.
proliferating stalk cells attract pericytes and deposit basement membranes to become stabilised. Myeloid cells expressing TIE-2 is also involved in this.


formin anastomosis between vessels

facilitated by vessel intercations with macriphages that can act as bridge cells that promote fillopodia contact between opposing tip cells.
upon contact, adhesion junctions are formed by VE-Cadherin, first at the tips of fillopodia, then extendidng along the nterface of contacting cells.

possible signalling pathways involve TIE2 receptor


models for lumen formation

1. intracellular vacuoles coalesce and connect with each other and vacuoles in the neighbouring cells
2. intercellular vacuel exocytosis into the extracellular space
lumnal repulsion by apical membrane


stabilisation and quescence

after fusion of neighbouring branches, lumen formation allows the perfusion of the neovessel which resumes the quiescence by promoting a phalanx phenotype, re-establishemnt of junctions, depositon of BM and pericyte maturatrion.


VE-cadherin and its regulation

•Ca+ dependent adhesion molecule at endothelial adherens junctions
•Homophilic interaction with VE-Cadherin on opposite cells
•Inside the cell, binds to catenins to mediate interaction with actin cytoskeleton and signalling
•Essential for vascular development: VE-Cad Ko mice die before birth
•Regulates permeability, survival, shear stress and growth factors signals

Regulation of VE-cadherin activity and junction stability through phosphorylation
(A)In quiescent endothelial monolayers, VE-cadherin clusters at junctions in zipper-like structures; p120, β-catenin and plakoglobin bind directly to VE-cadherin
(B) Phosphorylation of VE-cadherin reduces junctions’ strength and the VE-cadherin complex becomes partially disorganized: increased permeability, leukocyte transmigration


how do mural cells help to stabilise the neovessels>

Blood vessels in embryonic skin
Vessel stabilisation. Stalk cells (purple) recruit pericytes (orange) to stabilise the vasculature, possibly through the production of stabilising factors such as TIMP3 and ANG1.
ANG1 signalling through the TIE2 receptor stabilises the vasculature, in part via inducing DLL4 expression in the endothelial cells (ECs) and activating notch signalling.

Notch activation then plays a dual role in vascular stabilisation:

first, it downregulates VEGFR2 expression, thereby preventing further sprouting through activation of the VEGF/notch signalling pathway;

second, it induces the expression of NRARP, which promotes WNT signalling leading to increased proliferation and tight junction (TJ) stabilisation.


modes of angiogenesis

Vessel formation can occur by sprouting angiogenesis (a), by the recruitment of bone-marrow-derived and/or vascular-wall-resident endothelial progenitor cells (EPCs) that differentiate into endothelial cells (ECs; b), or by a process of vessel splitting known as intussusception (c). d–f,
Tumour cells can co-opt pre-existing vessels (d), or tumour vessels can be lined by tumour cells (vascular mimicry; e) or by endothelial cells, with cytogenetic abnormalities in their chromosomes, derived from putative cancer stem cells (f). Unlike normal tissues, which use sprouting angiogenesis, vasculogenesis and intussusception (a–c), tumours can use all six modes of vessel formation (a–f).


wnt signalling

? surival


TIE2 signalling system

•Ang-1 and Ang-2 are antagonistic ligands of the Tie2 receptor
•Ang-1 binding to Tie2 promotes vessel stability and inhibits inflammatory gene expression
•Ang-2 antagonises Ang-1 signalling, promotes vascular instability and VEGF-dependent angiogenesis

Endothelial activation denotes a key event in angiogenesis and vascular inflammation that causes increased responsiveness towards various pro-angiogenic or proinflammatory stimuli. In inflammation, endothelial activation is characterized by increased expression of luminal adhesion molecules, leukocyte recruitment, and altered vasomotor tone, resulting in vascular barrier breakdown. The endothelial-specific angiopoietin–Tie ligand–receptor system has recently emerged as a non-redundant regulator of endothelial activation.

Angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) are antagonistic ligands that bind to the extracellular domain of the Tie2 receptor, which is almost exclusively expressed by endothelial cells.

Binding of Ang-1 to Tie2 promotes vessel integrity, inhibits vascular leakage and suppresses inflammatory gene expression.

Ang-2 is stored in Weibel–Palade bodies and is rapidly secreted and induced upon stimulation, whereas Ang-1 is constitutively expressed by pericytes and vascular smooth muscle cells.

Binding of antagonistic Ang-2 completely disrupts protective Tie2 signalling in the majority of experimental studies.
The important role of the Ang-Tie system has also been demonstrated in several preclinical inflammatory models. Thus, either Ang-1 gain-of-function, or Ang-2 loss-of-function animals are protected from capillary leakage and subsequent end organ damage (i.e. lung injury, circulatory shock, tissue hypoperfusion).


TIE2 receptor functions

Very favourable and protective:
Leukocyte recruitment inhibition,
Promotes survival via PI3K/Akt
Inhibits NF-kB nad is anti-inflammatory
Inihbits Rho kinase and prevents junction destabilisation

these effects are promoteg by Angiipoietin-1 but antagonised by angiopoietin-2