Development of retinal vasculature is mediated by hypoxia-induced vascular endothelial growth factor (VEGF) expression by neuroglia

Abstract

We have studied the role of the hypoxia-inducible angiogenic growth factor vascular endothelial growth factor (VEGF) in the induction and control of vessel growth in the developing retina of rats and cats, using in situ hybridization techniques. VEGF is expressed successively in two layers of neural retina, the innermost (axon) layer and the inner nuclear layer (INL). In the axon layer, VEGF is expressed transiently by astrocytes as they spread across the layer, closely preceding the formation of superficial vessels. In the INL, VEGF is expressed transiently by somas at the middle of the layer (presumably Muller cells), closely preceding the formation of the deep layer of retinal vessels. We propose that hypoxia caused by the onset of neuronal activity is detected by strategically located populations of neuroglia, first astrocytes, then Muller cells. In response they secrete VEGF, inducing formation of the superficial and deep layers of retinal vessels, respectively. As the vessels become patent, they relieve the hypoxic stimulus, so vessel formation is matched to oxygen demand. This hypothesis was tested experimentally in three ways. Expression of the high affinity flk-1 receptor for VEGF was demonstrated in newly formed retinal vessels, confirming that the secreted VEGF acts on the vessels, in a paracrine fashion. Direct hypoxic regulation of VEGF expression by macroglia was demonstrated in primary cultures of astrocytes and in cells of a glioma line. Hypoxic regulation of VEGF expression in the intact developing retina was demonstrated by showing that oxygen-enriched atmospheres that inhibit vessel formation also suppress endogenous VEGF production.

Vascular endothelial growth factor in eye disease

“…These include the retinal pigmented epithelium (RPE) (Miller et al, 1997), astrocytes (Stone et al, 1995), Müller cells (Robbins et al, 1997), vascular endothelium (Aiello et al, 1995) and ganglion cells (Ida et al, 2003). These cells differ widely in their responses to hypoxia; in vitro studies show that Müller cells and astrocytes generally produce the greatest amounts of VEGF under hypoxic conditions (Morrison et al, 2007;Aiello et al, 1995;Hata et al, 1995).…”

“…The process of normal vascularization has been extensively examined in the kitten, mouse and rat retinas (Ashton et al, 1957;Ashton, 1961;Ashton, 1970;Blanks and Johnson, 1983;Connolly et al, 1988;Chan-Ling et al, 1990;Smith et al, 1994;Stone et al, 1995;Dorrell and Friedlander, 2006). Several studies have also examined the process in the human fetal retina (Michaelson, 1948;Nilausen, 1958;Cogan, 1963;Ashton, 1970;Nishimura and Taniguchi, 1982;Penfold et al, 1990;Gariano et al, 1994;Hughes et al, 2000).…”

“…Development of the retinal vasculature follows a common pattern in all species (Dorrell and Friedlander, 2006), but there are some dissimilarities as well (Ashton, 1968;Gariano et al, 1994). For example, in humans the process occurs primarily during the latter half of gestation, whereas in rodents the process is completed in the first two weeks after birth (Michaelson et al, 1954;Ashton, 1970;Stone et al, 1995;Engerman and Meyer, 1965). Generally, the completion of retinal vascular development is coincident with eye opening in mammalian species.…”

“…This network forms the scaffold upon which the primary vascular plexus is formed (Ling and Stone, 1988;Fruttiger et al,1996). Astrocytes express VEGF, which promotes endothelial cell proliferation and migration within this superficial plane (Stone et al, 1995). The formation of this primary vascular plexus begins in the region around the optic disc at the base of the hyaloid artery.…”

“…The "physiologic hypoxia" created by the increasing metabolic demands of the developing neural components of a largely avascular retina is the driving mechanism for this angiogenic growth. Low tissue PO 2 , hypoxia, results in secretion of retinal VEGF, and this leads to additional elaboration of the superficial vessel network and development of the deep network (Chan-Ling et al, 1990;Chan-Ling et al, 1995b;Stone et al, 1995;Zhang and Stone, 1997;Salhia et al, 2000). It is interesting to speculate that, because of their spatial distributions, astrocytes control the final growth phase and refinement of the superficial capillary network, while Müller cells drive growth of the deep network.…”