Molecular Vision 2006; 12:405-14 <http://www.molvis.org/molvis/v12/a47/> Received 15 April 2005 | Accepted 10 April 2006 | Published 20 April 2006 Proliferative diabetic retinal disease is a major cause of blindness in the Western world [1]. Microvascular occlusion leads to both hypoxia and ischemia due to nonperfusion of the inner retina [2-4]. The hypoxic retina responds by generating growth factors, including vascular endothelial growth factor (VEGF), culminating in excessive proliferation of new blood vessels within the retina, often extending into the vitreous [5- 8]. The conversion of arachidonic acid to prostaglandin H 2 (PGH 2 ) by the cyclooxygenase (COX) enzymes is the rate- limiting reaction in the synthesis of prostaglandins [9]. The COX enzymes exist as two distinct isoforms, COX-1 and COX-2. COX-1 is largely a constitutive isoform, whereas COX-2 is induced in response to many different stimuli [9], including ischemia. The products of these enzymes, the pros- taglandins, influence a wide range of biological processes of relevance to ischemic proliferative retinopathies. These include vascular tone, platelet activity, inflammation, and angiogen- esis [10,11]. Inhibitors of COX suppress angiogenesis in in vitro and in vivo models of excess neovascularization [12-15]. Both COX isoforms have been implicated as signals for angiogenic factors (VEGF, integrin engagement) and as inducers of VEGF expression [16,17]. COX-2 has also been reported to drive the angiogenic response in ocular vascular beds. Disruption of the COX-2 gene inhibits inflammation-induced corneal an- giogenesis, but not that induced by VEGF [18]. COX-1 is also reported to regulate the response of endothelial cells to angio- genic factors [19]. Both COX isoforms are expressed in mouse, rat, and human retina [20]. Recent studies have shown that selective and nonselective COX-inhibitors attenuate retinal neovascularization in a mouse model of oxygen-induced ret- inopathy [21-23]. The aim of the present study was to examine the contri- bution of each COX isozyme to retinal ischemia and neovascularization by using mice with specific disruption of either the COX-1 or COX-2 isoform in a model of prolifera- tive retinopathy. This approach allows the evaluation of the role of each isoform in oxygen-induced retinopathy without the confounding nonspecific effects of pharmacological COX inhibitors [24]. METHODS Murine model of oxygen-induced retinopathy: C57BL/6J x 129/Ola mice with a targeted disruption of either the COX-1 gene (COX-1 -/- ) or the COX-2 gene (COX-2 -/- ) were gener- ously provided by R. Langenbach (Laboratory of Environ- mental Carcinogenesis and Mutagenesis, National Institute of Environmental Health Sciences, National Institute of Health, Triangle Park, NC) [25,26]. The mice were employed in an established model of oxygen-induced retinopathy [27]. Wild- type mice used in these experiments were of the same genetic background (C57BL/6J x 129/Ola). Litters of 7-day-old mice ©2006 Molecular Vision COX-2 protects against thrombosis of the retinal vasculature in a mouse model of proliferative retinopathy Lorna M. Cryan, 1 Graham P. Pidgeon, 1 Desmond J. Fitzgerald, 2 Colm J. O’Brien 3 1 Department of Clinical Pharmacology, Royal College of Surgeons in Ireland, Dublin, Ireland; Departments of 2 Molecular Medi- cine and 3 Ophthalmology, Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin, Ireland Purpose: Cyclooxygenases (COX-1 and COX-2) and prostaglandins regulate angiogenesis in several settings, including cancer and ischemia. In the eye, both selective inhibitors of COX-2 and nonselective COX inhibitors are reported to suppress ischemia-related retinal angiogenesis. Such studies however, may be confounded by the nonspecific effects of inhibitors. Methods: Mice lacking either the COX-1 (COX-1 -/- ) or COX-2 isoform (COX-2 -/- ) were employed in a model of oxygen- induced retinopathy. Vascular responses were examined by histology, isolectin B4 staining of the abluminal endothelium, and retinal fluorescein angiography. Results: There was an increase in intravitreal endothelial nuclei in hyperoxia-treated mice compared to normoxic controls irrespective of the genotype. Quantitative analysis of fluorescein-perfused and isolectin B4-stained retinal angiograms at postnatal day 18 (P18) revealed similar global levels of neovascular tufts in hyperoxia-treated wild-type, COX-1 -/- , and COX-2 -/- mice. However, hyperoxia-treated COX-2 -/- mice had increased areas of retinal nonperfusion (29.2±1.9 com- pared to 16.3±2.7; n=6; p<0.001). COX-1 disruption had no effect (15.6±2.6; n=8). Platelet deposition within retinal vessels was increased in hyperoxia-treated COX-2 -/- mice (p<0.05). Conclusions: Genetic disruption of a single COX isoform is not sufficient to prevent oxygen-induced retinopathy. COX- 2 protects retinal vessels from thrombosis, limiting the area of retinal nonperfusion in oxygen-induced retinopathy. Correspondence to: Colm O’Brien, MD, Institute of Ophthalmol- ogy, Mater Misericordiae Hospital, 60 Eccles Street, Dublin 7, Ire- land; Phone: 353-1-8386730; FAX: 353-1-8305693; email: cobrien@mater.ie 405