Itzhak Hemo

Little is known about how the initial endothelial plexus is remodelled into a mature and functioning vascular network. Studying postnatal remodelling of the retina vasculature, we show that a critical step in vascular maturation, namely pericyte recruitment, proceeds by outmigration of cells positive for (alpha)-smooth muscle actin from arterioles and that coverage of primary and smaller branches lags many days behind formation of the endothelial plexus. The transient existence of a pericyte-free endothelial plexus coincides temporally and spatially with the process of hyperoxia-induced vascular pruning, which is a mechanism for fine tuning of vascular density according to available oxygen. Acquisition of a pericyte coating marks the end of this plasticity window. To substantiate that association with pericytes stabilizes the vasculature, endothelial-pericyte associations were disrupted by intraocular injection of PDGF-BB. Ectopic PDGF-BB caused the detachment of PDGF-beta receptor-positive pericytes from newly coated vessels, presumably through interference with endogenous cues, but had no effect on mature vessels. Disruption of endothelial-pericyte associations resulted in excessive regression of vascular loops and abnormal remodelling. Conversely, intraocular injection of VEGF accelerated pericyte coverage of the preformed endothelial plexus, thereby revealing a novel function of this pleiotropic angiogenic growth factor. These findings also provide a cellular basis for clinical observations that vascular regression in premature neonates subjected to oxygen therapy [i.e. in retinopathy of prematurity] drops precipitously upon maturation of retina vessels and a mechanistic explanation to our previous findings that VEGF can rescue immature vessels from hyperoxia-induced regression.

BACKGROUND: It is generally assumed that unwarranted, excessive neovascularization of the retina and iris is a direct response to a hypoxic retinal environment. Prompted by our previous findings that the potent angiogenic factor, vascular endothelial growth factor (VEGF), is hypoxia-inducible, we used in situ hybridization techniques to examine the thesis that VEGF functions as the link between retinal ischemia and a pathologic, intraocular, angiogenic response. EXPERIMENTAL DESIGN: To gain molecular access to human material representing progressive stages of angiogenic eye diseases, in situ hybridization analysis was carried out on sections of whole globes enucleated at the time of ongoing neovascularization. This methodology identified cells that have up-regulated VEGF expression during natural progression of the indicated diseases. A rabbit model was also used to determine whether experimentally induced retinal ischemia leads to up-regulation of VEGF expression. RESULTS: Proliferation of vascular elements in proliferative diabetic retinopathy and neovascularization of the retina and/or iris secondary to central retinal vein occlusion, retinal detachment, and intraocular tumors were always accompanied by induction of retinal VEGF expression. Furthermore, in each case, expression of VEGF was induced only in a particular layer of the retina (either the outer nuclear layer, the inner nuclear layer, or the ganglion cell layer), matching the zones affected by impaired perfusion. In a rabbit model simulating retinal vein occlusion, elevated levels of VEGF mRNA were detected within a few days of experimental induction of retinal ischemia, exclusively in the ischemic region. CONCLUSIONS: VEGF may be one of the long anticipated factors linking retinal ischemia and intraocular angiogenesis. Irrespective of the cause of retinal ischemia, sustained overproduction of VEGF by ischemic retinal cells may promote retinal and iris neovascularization in a number of neovascular eye diseases.