Endothelial Caspase-9 Activity Exacerbates Edema and Neuronal Dysfunction after Retinal Vein Occlusion

The retina is one of the most metabolically active tissue in the body, rendering it sensitive to vascular dysfunction. Consequently, diseases that disrupt normal retinal blood supply, including retinal vein occlusions (RVO) and diabetic retinopathy, are the leading causes of blindness in working-age adults. Despite available therapies, an estimated 50% of patients do not respond to treatment. We employed a mouse model of retinal vein occlusion (RVO), achieved by tail-vein injection of rose bengal, followed by laser photocoagulation of retinal veins. In vivo analyses – optical coherence tomography (OCT), fluorescein angiography, and electroretinograms (ERGs) - were conducted with the Micron IV system (Phoenix Research Labs). RVO induces acute retinal edema, which peaks during the first 24 hours following injury. Over a 7 day time course the edema resolves, revealing a permanent retinal thinning due to death of retinal neurons. We identified caspase-9, a protease traditionally associated with apoptosis, as an essential mediator of edema. Increased levels of activated caspase-9 were detected in vascular endothelial cells 1 hour following RVO. We tested RVO in mice with inducible endothelial-cell-specific deletion of caspase-9 (iC9 ECKO). Compared to littermate controls, iC9 ECKO mice develop less edema, and sustain less retinal degeneration after RVO injury. ERG analysis showed preservation of retinal function in iC9 ECKO mice. To study whether inhibiting caspase-9 would provide protection against RVO we utilized a highly specific caspase-9 inhibitor, which we can deliver to the retina using simple eyedrops. Treatment of wildtype mice with the caspase-9 inhibitor immediately after induction of RVO provided morphologic, biochemical and functional retinal protection. Inhibition of caspase-9 reduces edema, protects retinal morphology, and helps prevent vision loss following RVO injury. Our studies indicate that endothelial caspase-9 plays an essential role in regulating edema pathogenesis. Moreover, our novel cell permeant caspase-9 inhibitor abrogates the edema and may be a potential therapy for individuals suffering from vascular eye disease