Reduced redox state allows prolonged survival of axotomized neonatal retinal ganglion cells

Axonal injury to CNS neurons results in apoptotic cell death. The processes by which axotomy signals apoptosis are diverse, and may include deprivation of target-derived factors, induction of injury factors, bursts of reactive oxygen species (ROS), and other mechanisms. Our previous studies demonstrated that death of a dissociated retinal ganglion cell, an identified CNS neuron, is ROS-dependent. To better define the mechanisms by which ROS induce retinal ganglion cell death after axotomy, we studied their effects in dissociated neonatal rat retinal cultures. Postnatal day 2-4 Long-Evans rat retinal ganglion cells were retrogradely labeled with the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI). Postnatal day 7-9 retinas were dissociated and cultured in the presence of specific ROS generating systems, scavengers, or redox modulators. Retinal ganglion cells were identified by DiI positivity and viability determined by metabolism of calcein-acetoxymethyl ester. We found that ROS scavengers protected against retinal ganglion cell death after acute dissociation, and the effects of ROS appeared to be due to shifts in the redox potential, as retinal ganglion cell survival was critically dependent on redox state, with greatest survival under mildly reducing conditions. Culture of retinal ganglion cell with the non-thiol-containing reducing agent tris(carboxyethyl)phosphine resulted in long-term survival equivalent to or better than with neurotrophic factors. Our data suggest that axotomy-associated neuronal death induced by acute dissociation may be partly dependent on ROS production, acting to shift the redox state and oxidize one or more key thiols. Understanding the mechanisms by which ROS signal neuronal death could result in strategies for increasing their long-term survival after axonal injury.