Visualization of gene expression in whole mouse retina by in situ hybridization.

The mouse retinal vasculature provides a powerful model system for studying development and pathologies of the vasculature. Because it forms as a two-dimensional flat plexus, it is easily imaged in its entirety in whole-mount retinal preparations. In order to study molecular signaling mechanisms, it is useful to visualize the expression of specific genes in the entire vascular plexus and retina. However, in situ hybridization on whole-mount retinal preparations is problematic because isolated retinas have a tendency to curl up during hybridization and are difficult to stain. Here we provide a detailed protocol that overcomes these difficulties and visualizes the mRNA distribution of one or two genes in the context of the counterstained retinal vasculature. The protocol takes 3-4 d for single-probe stains, with an additional 2 d for immunohistochemistry co-labeling. In situ hybridization with two probes adds a further 3 d.

Apelin is required for non-neovascular remodeling in the retina.

Retinal pathologies are frequently accompanied by retinal vascular responses, including the formation of new vessels by angiogenesis (neovascularization). Pathological vascular changes may also include less well characterized traits of vascular remodeling that are non-neovascular, such as vessel pruning and the emergence of dilated and tortuous vessel phenotypes (telangiectasis). The molecular mechanisms underlying neovascular growth versus non-neovascular remodeling are poorly understood. We therefore undertook to identify novel regulators of non-neovascular remodeling in the retina by using the dystrophic Royal College of Surgeons (RCS) rat and the retinal dystrophy 1 (RD1) mouse, both of which display pronounced non-neovascular remodeling. Gene expression profiling of isolated retinal vessels from these mutant rodent models and wild-type controls revealed 60 differentially expressed genes. These included the genes for apelin (Apln) and for its receptor (Aplnr), both of which were strongly up-regulated in the mutants. Crossing RD1 mice into an Apln-null background substantially reduced vascular telangiectasia. In contrast, Apln gene deletion had no effect in two models of neovascular pathology [laser-induced choroidal neovascularization and the very low density lipoprotein receptor (Vldlr)-knockout mouse]. These findings suggest that in these models apelin has minimal effect on sprouting retinal angiogenesis, but contributes significantly to pathogenic non-neovascular remodeling.

Astrocyte-derived vascular endothelial growth factor stabilizes vessels in the developing retinal vasculature.

Vascular endothelial growth factor (VEGF) plays a critical role in normal development as well as retinal vasculature disease. During retinal vascularization, VEGF is most strongly expressed by not yet vascularized retinal astrocytes, but also by retinal astrocytes within the developing vascular plexus, suggesting a role for retinal astrocyte-derived VEGF in angiogenesis and vessel network maturation. To test the role of astrocyte-derived VEGF, we used Cre-lox technology in mice to delete VEGF in retinal astrocytes during development. Surprisingly, this only had a minor impact on retinal vasculature development, with only small decreases in plexus spreading, endothelial cell proliferation and survival observed. In contrast, astrocyte VEGF deletion had more pronounced effects on hyperoxia-induced vaso-obliteration and led to the regression of smooth muscle cell-coated radial arteries and veins, which are usually resistant to the vessel-collapsing effects of hyperoxia. These results suggest that VEGF production from retinal astrocytes is relatively dispensable during development, but performs vessel stabilizing functions in the retinal vasculature and might be relevant for retinopathy of prematurity in humans.