Dexras1 Mediates Retinal Ganglion Cell Loss Induced by NMDA Excitotoxicity

Abstract

Purpose: Excitotoxicity has been implicated in retinal ganglion cell (RGC) loss in optic neuropathies such as glaucoma, but mechanisms by which overstimulation of glutamate receptors leads to neuronal damage are not well understood. Recent studies identified an intracellular signaling pathway that may be involved in glutamate excitotoxicity. Stimulation of NMDA-type glutamate receptors leads to increased neuronal nitrous oxide synthase which nitrosylates the dexamethasone-induced G protein Dexras1. Dexras1 subsequently interacts with the divalent metal transporter DMT1 which imports iron into the cell leading to iron-induced oxidative stress. We investigated whether Dextras1 mediates NMDA-induced loss of RGCs.

Methods: RGCs of mice containing a targeted deletion of dexras1 (Dexras1 KO) and wild-type control mice were retrogradely-labeled with fluorogold by injection into superior colliculi. One week later, NMDA (200nM) was injected intravitreally into one eye and PBS was injected in the contralateral eye of each mouse. Following sacrifice, retinas were flat mounted on glass slides, viewed by fluorescence microscopy, and photographed. RGCs were quantified by counting fluorescent cells in 12 standardized fields of each isolated retina.

Results: Mice were sacrificed 5 days following NMDA administration. NMDA induced a 60% reduction in numbers of RGCs (386±76 RGCs/retina vs 979±88 RGCs/retina in PBS injected eyes) in wild-type mouse eyes. Retinas from Dexras1 KO mice exposed to NMDA had significantly higher numbers of RGCs (p < 0.001). In Dexras1 KO mice, there was a small trend toward RGC loss in NMDA injected eyes (859±62 RGCs/retina) as compared to PBS injected eyes (991±64 RGCs/retina) that was not statistically significant.

Conclusions: NMDA-induced loss of RGCs is almost completely blocked in the absence of Dexras1, suggesting that Dexras1 plays a critical role in mediating RGC excitotoxicity. Signaling through Dexras1 drives iron uptake through the DMT1 iron importer in other cell types, and results here suggest further studies of this pathway in RGCs may provide insight into mechanisms of glutamate excitotoxicity and identify molecular targets for neuroprotective therapies.

Keywords: 531 ganglion cells • 613 neuro-ophthalmology: optic nerve