Cell-autonomous generation of mitochondrial superoxide is a signal for cell death in differentiated neuronal precursor cells

The vast majority of optic neuropathies result from retinal ganglion cell (RGC) axonal injury. This induces cell death and is associated with a burst of mitochondria-generated superoxide within the soma. It is unclear whether there is a clear causal relationship between superoxide generation and cell death. To determine whether mitochondrial-generated superoxide can cause cell-autonomous death signaling, we knocked down SOD2 in a pure population of RGC-5 cells, a neuronal precursor cell line that can be differentiated to resemble retinal ganglion cells. RGC-5 cells were differentiated and transfected with siRNA for SOD2 or a scramble control. Viability, superoxide production, cytotoxic RNA transfection efficiency, and measurement of SOD2 protein levels by immunoblotting were assayed at varying times after transfection. SOD2 knockdown increased intracellular superoxide levels and cell death was presumed triggered from knockdown. This was amplified when extramitochondrial superoxide was elevated with the redox cycling agent menadione. Dysregulation of mitochondrial superoxide in differentiated RGC-5 cells is likely a potent signal for cell death, consistent with a role of this reactive oxygen species in apoptosis signaling after axonal injury.