Dual Effects of Oxygen Supplement in Ischemia-driven Angiogenesis: Accelerating Repair while Preventing Pathology

Abstract

Purpose: : Physiological angiogenesis is critical to vascular development and tissue repair. In contrast, pathological angiogenesis is linked to diseases such as proliferative diabetic retinopathy and retinopathy of prematurity. In both processes, ischemia-driven VEGF expression is believed to act as a key mechanism. Here we determine the impact of oxygen supplement (OS) in a mouse model of ischemia-driven pathological angiogenesis.

Methods: : Neonatal mice were maintained in 75% oxygen for 5 days (P7-P12). This causes vaso-obliteration which results in ischemic retinopathy when pups are returned to room air and triggers retinal pathological neovascularization (PN). To investigate the effect of OS, 75% oxygen was delivered during the ischemic phase of the retinopathy, either before or after the development of PN.

Results: : PN area was decreased by 92% by OS from P14 to P20 (P<0.001). In contrast, OS significantly increased retinal vascular coverage from 77% to 88%, indicating an accelerated physiological angiogenesis. To address potential mechanisms, retinal morphology was analyzed in mice receiving shorter OS (P14 to P17). Similarly, PN was eliminated by OS (P<0.001). Physiological angiogenesis was not enhanced, but formation of capillary endothelial tip cells was significantly increased and associated with recovery of the astrocyte network that forms the vascular bed. Analysis of expression of angiogenic factors (VEGF, EPO, bFGF, PDGF, angiopoiten-2, IGF-1) revealed that their mRNA levels were significantly increased in ischemic retinas but were normalized by OS (P<0.001). Blocking VEGF with sFlt-1 abolished PN but impaired revascularization in ischemic retina. Injecting VEGF did not induce PN in oxygen supplemented mice, suggesting VEGF is necessary but not sufficient to cause pathological angiogenesis. Neovascular tufts showed normal mural cell coverage by NG2 staining. However, brief OS (P17 to P18) resulted in neovascular tuft regression as determined by confocal imaging.

Conclusions: : These data support the concept that OS may be clinically useful to abolish pathological angiogenesis while increasing physiological vascular recovery in ischemic retinal tissues. The beneficial effect of oxygen may be attributable to multiple actions in correcting a range of biochemical and cellular abnormalities.

Keywords: hypoxia • retinopathy of prematurity • oxygen