Cellular and Molecular Mechanisms of Retinal Ganglion Cell Death in Hypoxic-Ischemic Injuries

Loss of retinal ganglion cells (RGCs) occurs in retinal degenerative diseases, such as glaucoma, age-related macular degeneration, diabetic retinopathy, central retinal artery occlusion and ischemic central retinal vein thrombosis in adults and in retinopathy of prematurity in infants. A critical role of hypoxia, which underlies most of the above disorders, has been reported in causing RGC death. Disruption of blood-retinal barrier (BRB) occurs due to increased production of vascular endothelial growth factor and nitric oxide in response to hypoxia resulting in extravasation of serum derived substances and retinal edema and this may be one of important factors mediating RGC death. Activation of microglial cells in response to hypoxia and subsequent increased release of proinflammatory cytokines by them such as tumor-necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) target the TNF-receptor 1 and IL-receptor 1 on RGCs. Excess release of glutamate in retinal tissue under hypoxia activates the ionotropic glutamate receptors causing excitotoxicity through increased influx of calcium into the RGCs. Increased production of TNF-α, IL-1β and enhanced intracellular calcium results in RGC death through activating several pathways such as caspase signaling, mitochondrial dysfunction and oxidative stress. In our investigations, melatonin, an antioxidant, was shown to reduce RGC death in the adult and neonatal hypoxic retina, through suppression of TNF-α, IL-1β and glutamate levels as well as oxidative stress. Moreover, the function and structure of BRB was well preserved in these animals along with reduction in retinal edema. In view of our observations, we suggest that melatonin could be considered as a therapeutic agent to reduce RGC death in various retinal pathologies, in addition to other potential drugs/molecules of therapeutic interest that could render protection to RGCs. However, future in-depth research on the effects of melatonin on the retina under hypoxic conditions needs to be undertaken to explore its full potential in preventing/ameliorating RGC death.