ANGIOGENESIS

Vasculogenesis: Embryonic development from endothelial precursors called ‘angioblasts’

Angiogenesis/ neovascularization:

•Process of blood vessel formation in adults • By branching and extension of adjacent blood vessels and • Recruitment of EPCs from bone marrow.

This is Critical in chronic inflammation and fibrosis , tumor growth and vascularization of ischemic tissues. •Exploration of therapeutic potential of agents that are :

• Pro-angiogenic (increase blood vessels)• Anti-angiogenic (Block pathologic angiogenesis)

Angiogenesis from Pre-Existing vessels:• Vasodilation in response to NO and VEGF• Proteolytic degradation of BM of parent vessels by metalloprotinases and disruption

of cell to cell contact between endothelial cells of vessels by plasminogen activators . • Migration of endothelial cells toward angiogenic sitmulus.• Proliferation of endothelial cells behind leading front of migrating cells.• Maturation of endothelial cells which included inhibition of growth and remodeling

into capillary tubes. • Recruitment of peri-endothelial cells. (pericytes and vascular smooth muscle cells. )

Angiogenesis from EPCs:Embryonic development of hematopoietic and vascular systems have common precursor ‘hemangioblast’

• Hematopoietic Stem cells • Angioblasts.

• Angioblast like cells ‘EPCs’ • Homing mechanism uncertain.• Express marker of both hematopoietic stem cells and endothelial specific.• Participate in replacement of lost endothelial cells, reendothelization of vascular

implants, neovascularization of ischemic organs.

GROWTH FACTORS AND RECEPTORS INVOLVED IN ANGIOGENESIS:

•VEGF : Secreted by many mesenchymal cells and stromal cells.•VEGFR-2, tyrosine kinase receptors restricted to endothelial cells. •VEGF Interaction and mobilization of EPCs Angiogenesis •VEGF Pre-Existing local vessels sprouting of new capillaries•FGF2 Endothelial proliferation, differentiation and migration.

Vascular Endothelial Growth Factors (VEGF)

Proteins Family members: VEGF (VEGF-A), VEGF-B, VEGF-C, VEGF-DDimeric glycoprotein with multiple isoformsTargeted mutations in VEGF result in defective vasculogenesis and angiogenesis.

Production Expressed at low levels in a variety of adult tissues and at higher levels in a few sites, such as podocytes in the glomerulus and cardiac myocytes

Inducing Agents

HypoxiaTGF-βPDGFTGF-α

Receptors VEGFR-1VEGFR-2VEGFR-3 (lymphatic endothelial cells)Targeted mutations in the receptors result in lack of vasculogenesis

Functions Promotes angiogenesisIncreases vascular permeabilityStimulates endothelial cell migrationStimulates endothelial cell proliferationVEGF-C selectively induces hyperplasia of lymphatic vasculatureUp-regulates endothelial expression of plasminogen activator, plasminogen activator inhibitor 1, and collagenase

Modulation of Vasculogenisis:• Notch pathway: Promotes proper branching of

new vessels and prevents excessive angiogenesis by decreasing responsiveness to VEGF.

• Notch ligands and receptors are membrane bound molecules conserve in species.

• 5 notch ligands in mammals : jagged 1&2, delta like ligands (DLL) 1,3,4

• 4 trans-membrane receptors: Notch 1-4• DLL4 is endothelial cells specific and is expressed

in arteries and capillaries but not in veins.

• Newly formed vessels are fragile, needs stabilization requiring pericytes and smooth muscles cells.

• Ang 1 and Ang 2 , PDGF, TGF- β participate in stabilization process. – Ang1 Tie 2; to recruit periendothelial cells. – PDGF recruit smooth muscles cells– TGF β production of ECM proteins

•Physiologic and pathologic angiogenesis can be influences by agents or conditions that stimulates VEGF expression e.g. (TGF β, PDGF. TGF- α ) and tissue hypoxia.

ECM proteins as regulator of angiogeneisis:• Mortality and directed migration of endothelial

cells is required for the formation of new-blood vessels. These processes are controlled by several classes of proteins.

• Integrins αv β3 – Multiple effects, • Matricellular proteins - destabilize cells – matrix

interactions and therefore promote angiogenesis.

• Proteinases and matrix metalloproteinases.

Endothelial Cells:Endothelium: • Single cell thick continuous lining of the entire

cardiovascular system. Its structure and function is vital for homeostasis and normal circulatory function. Endothelial cells can be identified, immuno histochemically with antibiodies to PECAM-I, CD34, vWF.

• Endothelium has many synthetic and metabolic properties.

MAINTENANCE OF PERMEABILITY BARRIER ELABORATION OF ANTICOAGULANT, ANTITHROMBOTIC, FIBRINOLYTIC REGULATORS

• Prostacyclin • Thrombomodulin • Heparin-like molecules • Plasminogen activator

ELABORATION OF PROTHROMBOTIC MOLECULES • Von Willebrand's factor • Tissue factor • Plasminogen activator inhibitor EXTRACELLULAR MATRIX PRODUCTION (COLLAGEN, PROTEOGLYCANS) MODULATION OF BLOOD FLOW AND VASCULAR REACTIVITY • Vasconstrictors: endothelin, ACE • Vasodilators: NO, prostacyclin REGULATION OF INFLAMMATION AND IMMUNITY • IL-1, IL-6, chemokines • Adhesion molecules: VCAM-1, ICAM, E-selectin, P-selectin • Histocompatibility antigens

REGULATION OF CELL GROWTH • Growth stimulators: PDGF, CSF, FGF • Growth inhibitors: heparin, TGF-β OXIDATION OF LDL

ENDOTHELIAL CELLS PROPERTIES AND FUCNTIONS

• Semipermeable. • Vascular endothelium has phenotypic

variations.– Large vessels and capilaries– Arteries and veins – Lymphatics

• Structurally endothelial cell can respond to pathophysiological stimuli “Endothelium activation”

Age related Macular degeneration•Advancing age is a risk factor.•71% heritable, CFH (Complement factors H) CC genotype. •Structural and functional unit:

•RPE, •Bruch membrane, •Choroidal vasculature

Disturbance in any component of unit affect health of overlying photoreceptors producing visual loss.

•ARMD: •Atrophic or dry, •Neovascular or wet/exudative.

•Choroidal neovascularization:• Presence of angiogenic vessels presumably originate from choriocapillaris

and penetrate through the Bruck’s membrane beneath the RPE or penetrate RPE become situated beneath the neurosensory retina. These vessels leaks, excude, haemorrhage vitreous hemorrhage.

•Neovascular ARMD → VEGF antagonists into vitreous of the effected eye.

•Choroidal neovascular membrane also seen in- pathologic myopia, trauma to bruck’s membrane, angiods streaks, immunological response to systemic histoplasmosis.