Identification of a synergistic interaction between endothelial cells and retinal pigment epithelium.

The retinal pigment epithelium located between the neurosensory retina and the choroidal vasculature is critical for the function and maintenance of both the photoreceptors and underlying capillary endothelium. While the trophic role of retinal pigment epithelium on choroidal endothelial cells is well recognized, the existence of a reciprocal regulatory function of endothelial cells on retinal pigment epithelium cells remained to be fully characterized. Using a physiological long-term co-culture system, we determined the effect of retinal pigment epithelium-endothelial cell heterotypic interactions on cell survival, behaviour and matrix deposition. Human retinal pigment epithelium and endothelial cells were cultured on opposite sides of polyester transwells for up to 4 weeks in low serum conditions. Cell viability was quantified using a trypan blue assay. Cellular morphology was evaluated by H&E staining, S.E.M. and immunohistochemistry. Retinal pigment epithelium phagocytic function was examined using a fluorescent bead assay. Gene expression analysis was performed on both retinal pigment epithelium and endothelial cells by quantitative PCR. Quantification of extracellular matrix deposition was performed on decellularized transwells stained for collagen IV, fibronectin and fibrillin. Our results showed that presence of endothelial cells significantly improves retinal pigment epithelium maturation and function as indicated by the induction of visual cycle-associated genes, accumulation of a Bruch's membrane-like matrix and increase in retinal pigment epithelium phagocytic activity. Co-culture conditions led to increased expression of anti-angiogenic growth factors and receptors in both retinal pigment epithelium and endothelial cells compared to monoculture. Tube-formation assays confirmed that co-culture with retinal pigment epithelium significantly decreased the angiogenic phenotype of endothelial cells. These findings provide evidence of critical interdependent interactions between retinal pigment epithelium and endothelial cell involved in the maintenance of retinal homeostasis.

Fatty acid binding protein 4 deficiency protects against oxygen-induced retinopathy in mice.

Retinopathy of prematurity (ROP) is a leading cause of blindness in children worldwide due to increasing survival rates of premature infants. Initial suppression, followed by increased production of the retinal vascular endothelial growth factor-A (VEGF) expression are key events that trigger the pathological neovascularization in ROP. Fatty acid binding protein 4 (FABP4) is an intracellular lipid chaperone that is induced by VEGF in a subset of endothelial cells. FABP4 exhibits a pro-angiogenic function in cultured endothelial cells and in airway microvasculature, but whether it plays a role in modulation of retinal angiogenesis is not known. We hypothesized that FABP4 deficiency could ameliorate pathological retinal vascularization and investigated this hypothesis using a well-characterized mouse model of oxygen-induced retinopathy (OIR). We found that FABP4 was not expressed in retinal vessels, but was present in resident macrophages/microglial cells and endothelial cells of the hyaloid vasculature in the immature retina. While FABP4 expression was not required for normal development of retinal vessels, FABP4 expression was upregulated and localized to neovascular tufts in OIR. FABP4-/- mice demonstrated a significant decrease in neovessel formation as well as a significant improvement in physiological revascularization of the avascular retinal tissues. These alterations in retinal vasculature were accompanied by reduced endothelial cell proliferation, but no effect on apoptosis or macrophage/microglia recruitment. FABP4-/- OIR samples demonstrated decreased expression of genes involved in angiogenesis, such as Placental Growth Factor, and angiopoietin 2. Collectively, our findings suggest FABP4 as a potential target of pathologic retinal angiogenesis in proliferative retinopathies.

PGC-1α regulates normal and pathological angiogenesis in the retina.

Neovascular diseases of the eye are the most common causes of blindness worldwide. The mechanisms underlying pathological neovascularization in the retina remain incompletely understood. PGC-1α is a transcriptional coactivator that plays a central role in the regulation of cellular metabolism. In skeletal muscle, PGC-1α induces VEGFA expression and powerfully promotes angiogenesis, suggesting a similar role in other tissues. This study investigates the role of PGC-1α during normal and pathological vascularization in the retina. We show that PGC-1α induces the expression of VEGFA in numerous retinal cells, and that PGC-1α expression is strongly induced during postnatal retinal development, coincident with VEGFA expression and angiogenesis. PGC-1α(-/-) mice have a significant reduction of early retinal vascular outgrowth, and reduced density of capillaries and number of main arteries and veins as adults. In the oxygen-induced retinopathy model of retinopathy of prematurity, PGC-1α expression is dramatically induced in the inner nuclear layer of the retina, suggesting that PGC-1α drives pathological neovascularization. In support of this, PGC-1α(-/-) mice subjected to oxygen-induced retinopathy had decreased expression of VEGFA and were protected against pathological neovascularization. These results demonstrate that PGC-1α regulates VEGFA in the retina and is required for normal vessel development and for pathological neovascularization. The data highlight PGC-1α as a novel target in the treatment of neovascular diseases of the eye.