Lack of Spontaneous Neovascularization and Attenuated Response to Laser Induced Choroidal Neovascularization in the Eye of a Novel Line of Insulin–Like Growth Factor–I Transgenic Mice

Abstract

Abstract: : Purpose: Insulin–like growth factor (IGF)–I has been associated with the pathogenesis of ocular angiogenesis, partly through the induction of VEGF expression. In the present study, we characterize the ocular change in an IGF–I transgenic line and investigate the effect of IGF–I overexpression on the development of choroidal neovascularization (CNV) induced by laser photocoagulation. Methods: A designated line of transgenic mice that overexpresses IGF–I driven by a mouse IGF–II promoter (IGF–II/I) was used in these experiments. Genotyping and radioactive immunoassay confirmed transgenic mice were compared with age and sex matched wildtype littermates. Ophthalmoscopy, fluorescein angiography (FA) and histology were used to evaluate the posterior ocular development and possible vascular abnormality. IGF–I and VEGF mRNA expressions were examined using in situ hybridization. CNV was induced using a series of 4 green–diode laser photocoagulation (532nm, 0.05 second, 75µm, and 120mW) placed concentrically around the optic disc when the mice were three and half months old. Two weeks after the laser treatment, FA was performed before tissue collection. Animal eyes were then processed for histopathologic analysis with light microscopy. Results: Histologically, no retinal abnormality was noticed and no spontaneous retina/choroidal neovascularization was observed in the IGF–II/I transgenic mouse up to ten months old. In situ hybridization revealed increased IGF–I mRNA signals in the retina, primarily in the ganglion cell layer and around the retinal pigment epithelium layer. Preliminary study showed transient upregulation of VEGF mRNA in the transgenic retina only around two months old compared to the wildtype. FA revealed less subretinal leakage at the laser sites in transgenic mice two weeks after laser photocoagulation. Quantitative histopathologic analysis demonstrated that the mean CNV membrane thickness was significantly thinner (p<0.05) in the transgenics (48.2±11.3 µm) than that in the wildtype mice (65.8±28.5 µm). Conclusions: These data indicate that overexpression of IGF–I in the retina does not necessarily lead to diabetes–like retinopathy as occurred in another IGF–I transgenic line. IGF–II/I transgenic mice develop less subretinal CNV following laser treatment than their wildtype littermates. Potential mechanisms underlying this inhibitory effect on CNV in this animal model remain to be investigated.

Keywords: choroid: neovascularization • growth factors/growth factor receptors • transgenics/knock-outs