Dexras1 Deletion Attenuates Retinal Ganglion Cell (RGC) Loss and Preserves Vision in Experimental Optic Neuritis

Abstract

Purpose : Optic neuritis is an inflammatory optic neuropathy associated with multiple sclerosis (MS). Prior studies suggest oxidative stress mediates RGC loss during optic neuritis in the EAE model of MS. Elevated iron found in the brain of MS patients as well as in EAE mice, may contribute to oxidative stress. Dexras1, a small G protein, is activated by S-nitrosylation by nitric oxide (NO) produced by inducible nitric oxide synthase (NOS) in activated microglia/macrophages, or by neuronal NOS, leading to iron import via divalent metal transporter1. Thus, we hypothesized that Dexras1 exacerbates oxidative stress induced neurotoxicity via iron entry in experimental optic neuritis, and examined whether dexras1 deletion reduces RGC damage in EAE.

Methods : EAE was induced in Dexras1 KO and wild-type mice by immunization with myelin oligodendroglial glycoprotein peptide. One wild-type EAE group was treated daily with oral iron chelator deferiprone (DFP). Visual function was assessed by optokinetic responses (OKR) at baseline and weekly until sacrifice, 6 weeks post-immunization. Retinas and optic nerves were isolated. Inflammation was assessed by H&E and IBA1 staining, demyelination by Luxol Fast Blue staining, and axon loss by neurofilament staining of optic nerves. RGCs were immunolabeled with Brn3a antibodies to quantify RGC survival.

Results : Progressive decreases in OKR occurred in wild-type EAE mouse eyes; whereas, Dexras1 KO and DFP treated EAE mice had significantly less vision loss (p<0.01; N=8-10/treatment group). Dexras1 KO (p<0.001) and DFP treated (p<0.05) EAE mice also had significant attenuation of RGC and axonal loss compared to wild-type EAE mice. Dexras1 KO and DFP treatment did not significantly reduce inflammation, but showed a trend toward decreased demyelination in EAE mice.

Conclusions : The preservation of vision and attenuation of RGC loss in Dexras1 KO mice suggests that the NO activated Dexras1 signaling cascade which drives iron entry into cells is a potential mechanism of neuronal death in experimental optic neuritis. Improved vision and RGC survival with DFP treatment further suggests a role of iron overload exacerbating neuronal damage. Results suggest modulation of Dexras1 signaling and iron chelation are potential novel treatment strategies for optic neuritis that warrant further investigation.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.