Unique vascular phenotypes following over-expression of individual VEGFA isoforms from the developing lens

Abstract

Formation of a correctly organised vasculature and subsequently embryonic survival is critically dependent on the dosage and site-specific expression of VEGF. Murine VEGF exists in three common isoforms (viz. 120, 164 and 188 amino acids) having different organ specific distribution levels. Gene knock-in studies show that expression of any of the individual isoforms of VEGF extends survival until birth, although each is associated with distinct organ-specific abnormalities. Comparison of the effects of VEGF isoform expression is complicated by the general lethality of mis-expression, in addition to cumulative effects of adjacent tissues from the inappropriately patterned vasculature. Here we investigate the effects of over-expression of individual VEGFA isoforms from the lens-specific alphaA-Crystallin promoter and characterise their effects on the vessel morphology of the hyaloid and developing retinal vasculature. Since the hyaloid vasculature is an anatomically distinct, transient vasculature of the eye, comprising 3 cell types (endothelium, pericytes and macrophages) it is possible to more readily interpret the role of individual VEGF-A isoforms in vascular pattern formation in this model. The severity of the vascular phenotype, characterised by a hyperplastic hyaloid at E13.5 and subsequently retinal vascular patterning and ocular defects, is most severe in transgenics over-expressing the more diffusible forms of VEGFA (120 and 164), whereas in VEGFA(188) transgenics the hyaloid vascular defects partially resolve post-natally. The results of this study indicate that individual isoforms of VEGFA induce distinct vascular phenotypes in the eye during embryonic development and that their relative doses provide instructive cues for vascular patterning.

Dynamics of angiogenesis during murine retinal development: a coupledin vivoandin silicostudy

“…This result compares favourably with data from studies describing the RVP structure in transgenic mice [8,9], where removal of the VEGF-A gradient, by increased VEGF-A expression, reduces the extent of endothelial migration. In the in silico setting here, both chemotaxis and capillary branching are impaired, resulting in a vascular network of irregular shape with numerous unperfused islands of retinal tissue (figure 8e).…”

“…VEGF-A 164 is the major isoform responsible for the formation of the RVP [7,8], and its role has been investigated in a number of studies: intra-ocular injection of VEGF-A sequestering antibodies has been found to inhibit endothelial migration and delay plexus formation [6], as has removal of the VEGF gradient in the retina, via increased expression of VEGF-A in transgenic mouse models [8,9]. Haptotactic factors, expressed on astrocytes, are also known to be important, as inhibition of the ability of ECs to bind fibronectin, via intra-ocular injection of anti-integrin ab1 antibodies, prevents EC migration [6].…”

von Hippel-Lindau protein regulates transition from the fetal to the adult circulatory system in retina

“…Mice selectively expressing single isoforms of VEGF (VEGF 120 or VEGF 188 ) or overexpressing VEGF under the control of a lens-specific promoter show a persistence of hyaloid vessels as well as defective retinal vascularization, supporting the concept that spatial distribution of VEGF regulates the transition from the embryonic to the adult circulatory system in the retina (Mitchell et al, 2006;Stalmans et al, 2002). Previously, Lang and colleagues clearly demonstrated that macrophages mediate hyaloid vessel regression through the paracrine action of WNT7B (Lobov et al, 2005).…”

Maintaining transparency: A review of the developmental physiology and pathophysiology of two avascular tissues

“…This observation led to the hypothesis that VEGF-A from the lens attracts the capillaries of the TVL. Over expression of VEGF-A or splice variants of VEGF-A caused increased formation of capillaries around the lens, indicating that a high level of this mitogen is capable of promoting the proliferation of the endothelial cells of the TVL and the growth of new vessels from the TVL [6][7][8]. Direct evidence that endogenous VEGF-A from the lens is responsible for the normal development of the TVL is provided by unpublished studies, in which Vegfa is removed from the lens by conditional deletion early in lens formation (Garcia, Kamath, Ferrara and Beebe, unpublished data).…”