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Visible light (360-760 nm) entering the eye impinges on the many ganglion cell mitochondria in the non-myelinated part of their axons. The same light also disrupts isolated mitochondrial function in vitro and kills cells in culture with the blue light component being particularly lethal whereas red light has little effect. Significantly, a defined light insult only affects the survival of fibroblasts in vitro that contain functional mitochondria supporting the view that mitochondrial photosensitizers are influenced by light. Moreover, a blue light insult to cells in culture causes a change in mitochondrial structure and membrane potential and results in a release of cytochrome c. Blue light also causes an alteration in mitochondria located components of the OXPHOS (oxidative phosphorylation system). Complexes III and IV as well as complex V are significantly upregulated whereas complexes I and II are slightly but significantly up-and downregulated, respectively. Also, blue light causes Dexras1 and reactive oxygen species to be upregulated and for mitochondrial located apoptosis-inducing factor to be activated. A blue light detrimental insult to cells in culture does not involve the activation of caspases but is known to be attenuated by necrostatin-1, typical of a death mechanism named necroptosis.

Blue light affects mitochondrial function and also the development tight junctions between ARPE19 cells, which results in a loss of cell viability. Importantly, red light delivered after a blue light insult is significantly blunted. These findings argue for the therapeutic use of red light as a noninvasive procedure to attenuate insults caused by blue light and other insults to retinal pigment epithelial cell mitochondria that are likely to occur in age-related macular degeneration.

Blue light impinging on the many mitochondria associated with retinal ganglion cells (RGCs) in situ has the potential of eliciting necroptosis through an action on RIP1/RIP3 to stimulate RGC death in diseases like glaucoma and diabetic retinopathy. Cells in culture die when exposed to blue light. The death process is mitochondria-dependent and is known to involve a decrease in the production of ATP, a generation of ROS, the activation of poly-(ADP-ribose) polymerase, the stimulation of apoptosis-inducing factor (AIF) as well as the up-regulation of heme-oxygenase-1 (HO-1). Our present results show that blue light-induced activation of AIF is not directly linked with the stimulation of RIP1/RIP3. Down-regulation of RIP1/RIP3 did not influence AIF. AIF activation therefore appears to enhance the rate of necroptosis by a direct action on DNA breakdown, the end stage of necroptosis. This implies that silencing of AIF mRNA may provide a degree of protection to blue light insult. Also, necrostatin-1 attenuated an increased turnover of HO-1 mRNA caused by blue light to suggest an indirect inhibition of necroptosis, caused by the action of necrostatin-1 on RIP1/RIP3 to reduce oxidative stress. This is supported by the finding that gene silencing of RIP1 and RIP3 has no effect on HO-1. We therefore conclude that inhibitors of RIP kinase might be more specific than necrostatin-1 as a neuroprotective agent to blunt solely necroptosis caused by blue light.

Previous studies have shown that light impinging on the retina in situ has the capacity to kill neuronal and non-neuronal cells in vitro by interacting directly with mitochondrial constituents. A number of fluorophores are associated with mitochondria which can potentially absorb different wave-lengths of light, including cytochrome oxidase. The aim of the present study was to compare the death mechanism of a light insult to RGC-5 cells in culture with that of sodium azide. Sodium azide's main toxic action is in inhibiting the function of cytochrome oxidase in the mitochondrial electron transport chain. Our studies showed that light and sodium azide kill RGC-5 cells via different mechanisms although some similarities do occur. Both inducers of cell death caused the generation of reactive oxygen species (ROS), the expression of phosphatidylserine, the breakdown of DNA and the activation of p38 MAPK, resulting in its translocation from the nucleus to the cytoplasm. However, light-induced cell death occurs via necroptosis, in that it was inhibited by necrostatin-1 and was caspase-independent. This was not the case for sodium azide, where the death process was caspase-dependent, occurred via apoptosis and was unaffected by necrostatin-1. Moreover, light caused an activation of the apoptosis inducing factor (AIF), c-Jun, JNK and HO-1, but it did not affect alpha fodrin or caspase-3. In contrast, sodium azide caused the activation of alpha fodrin and the stimulation of caspase-3 content without influencing AIF, c-Jun, JNK or HO-1. Therefore we conclude that light does not have a specific action on cytochrome oxidase in mitochondria to cause cell death.

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