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-D Dimeric glycoprotein with multiple isoforms Targeted 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 Hypoxia TGF-β PDGF TGF-α Receptors VEGFR-1 VEGFR-2 VEGFR-3 (lymphatic endothelial cells) Targeted mutations in the receptors result in lack of vasculogenesis Functions Promotes angiogenesis Increases vascular permeability Stimulates endothelial cell migration Stimulates endothelial cell proliferation VEGF-C selectively induces hyperplasia of lymphatic vasculature Up-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 α v β 3 – 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.

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.