The Role of TGF-β Signaling in Development and Maintenance of the Ocular Vasculature

Two of the leading causes of blindness worldwide are diabetic retinopathy and age-related macular degeneration. Both diseases are associated with pathological changes in the ocular vasculature. In the pathogenesis of diabetic retinopathy, a proliferation of the retinal vessels can be observed whereas in the wet form of age-related macular degeneration the choroidal vessels penetrate the retinal pigment epithelium and grow into the retina. The molecular mechanisms leading to these changes are mostly unknown, however there is evidence, that TGF-β signaling is involved in the pathogenesis of these diseases. To study the exact role of TGF-β signaling during development and maintenance of the ocular vasculature, we generated mice with an ocular deletion of Tgfbr2, using CAGG Cre-ERTM mice with two floxed alleles for Tgfbr2, and activated the Cre recombinase by applying tamoxifen eye drops. The tamoxifen treatment via eye drops resulted in efficient activation of the Cre recombinase in all ocular tissue. When tamoxifen application started on postnatal day P4, the lack of Tgfbr2 in the eye caused a phenotype that mimics essential characteristics of diabetic retinopathy. We observed the formation of multiple microaneurysms, leaky capillaries and hemorrhages. Immunohistochemial staining could show that the retinal capillaries were no longer covered by pericytes, but were densely coated by vascular smooth muscle-like cells. The basal lamina between endothelial cells and pericytes was markedly thickened. In addition, we found microglia with a reactive phenotype in close association with the retinal vessels correlating with the induction of immunmodulating cytokines. Furthermore, we found an induction of angiogenic molecules and an accumulation of retinal HIF-1α which suggests local hypoxia. In the following, we could observe neuronal apoptosis and an impairment of sensory function and vitreal neovascularization which lead to the formation of vitreal membranes and finally retinal detachment. In addition to the already mentioned structural and functional changes that mimic the phenotype of diabetic retinopathy, we found that these mice also show choroidal neovascularization (CNV), alterations of the Bruch’s membrane and the formation of basal laminar as well as basal linear deposits. All findings are similar to those seen in the wet form of AMD. A later deletion of TGFβ-signaling (at P21, after development of the retinal vasculature) resulted in a normal retinal vasculature but still in the formation of CNV. To investigate which cell type is responsible for the choroidal neovascularization, we generated mice with a specific deletion of Tgfbr2 in cells of the retinal pigment epithelium and mice with a specific deletion in endothelial cells. The deletion of Tgfbr2 in the RPE did not result in structural changes e.g. choroidal neovascularization. However, mice with a deletion of TGF-β signaling in endothelial cells developed alterations in the Bruch’s membrane, hemorrhages and CNV. We conclude that the lack of ocular TGF-β signaling leads to a phenotype that shows characteristics of both diabetic retinopathy and age-related macular degeneration. These results strongly suggest that TGF-β signaling plays an important role in the molecular pathogenesis of these ocular diseases.