VEGF-induced choroidal damage in a murine model of retinal neovascularisation.

BACKGROUND/AIMS: Photoreceptor-specific upregulation of vascular endothelial growth factor (VEGF) in a transgenic mouse model (Kimba) of retinal neovascularisation induces retinal vascular damage which appears similar to that in diabetic retinopathy. Here we have determined whether the choroidal vasculature is also affected in Kimba. METHODS: Kimba mice were assessed with fundus fluorescein angiography for mild, moderate or severe retinal vascular leakage prior to preparation of choroidal corrosion casts for quantitative analysis using scanning electron microscopy. VEGF was located immunohistochemically. RESULTS: Choroidal abnormalities included microaneurysms, constriction, shrinkage and dropout in the capillaries and tortuosity and loops in the arteries and veins which were similar to those observed in corrosion casts of the human choroid in diabetes. Similar to human diabetes, choroidal neovascularisation was not observed. The severity of choroidal damage correlated with the extent of retinal vascular leakage. In addition to the expected presence of VEGF in photoreceptors, VEGF was also detected in the pigment epithelium and choroid in the transgenic mice. CONCLUSION: We show that elevated retinal VEGF levels trigger pathophysiological changes in the choroid. We suggest that therapies to prevent vascular damage in diabetes must target both the retinal and choroidal vasculatures.

EphA/ephrin-A interactions during optic nerve regeneration: restoration of topography and regulation of ephrin-A2 expression.

During visual system development, interactions between Eph tyrosine kinase receptors and their ligands, the ephrins, guide retinal ganglion cell (RGC) axons to their topographic targets in the optic tectum. Here we show that Eph/ephrin interactions are also involved in restoring topography during RGC axon regeneration in goldfish. Following optic nerve crush, EphA/ephrin-A interactions were blocked by intracranial injections of recombinant Eph receptor (EphA3-AP) or phospho-inositol phospholipase-C. Topographic errors with multiple inputs to some tectal loci were detected electrophysiologically and increased projections to caudal tectum demonstrated by RT-97 immunohistochemistry. In EphA3-AP-injected fish, ephrin-A2-expressing cells in the retino-recipient tectal layers were reduced in number compared to controls and their distribution was no longer graded. The findings, supported by in vitro studies, implicate EphA/ephrin-A interactions in restoring precise topography and in regulating ephrin-A2 expression during regeneration.

Training on a visual task improves the outcome of optic nerve regeneration.

Optic nerve regeneration in a lizard, Ctenophorus ornatus, is dysfunctional despite survival of most retinal ganglion cells and axon regeneration to the optic tectum. The regenerated retino-tectal projection at 6 months has crude topography but by 1 year is disordered; visually-elicited behavior is absent via the experimental eye. Here, we assess the influence of training on the outcome of optic nerve regeneration. Lizards were trained to catch prey presented within the monocular field of either eye. One optic nerve was then severed and visual stimulation resumed throughout regeneration. In the trained group, presentation was restricted to the eye undergoing optic nerve regeneration; for the untrained group, the unoperated eye was stimulated. Pupil responses returned in trained but not in untrained animals. At 1 year, trained animals oriented to and captured prey; untrained animals demonstrated minimal orienting and failed to capture prey. Regenerated retino-tectal projections were topographic in the trained but not in the untrained group as assessed by in vitro electrophysiological recording and by carbocyanine dye tracing. In vitro electrophysiological recording during application of neurotransmitter antagonists to the tectum revealed that the level of GABAergic inhibition was modest in trained animals but elevated in the untrained group; responses were mainly AMPA-mediated in both groups. We conclude that training improves the behavioral outcome of regeneration, presumably by stabilizing and refining the transient retino-tectal map and preventing a build-up of tectal inhibition. The results suggest that for successful central nerve regeneration to occur in mammals, it may be necessary to introduce training to complement procedures stimulating axon regeneration.