Glaucoma is a neurodegenerative disease of the eye and it is one of the leading causes of blindness. Glaucoma is characterized by progressive degeneration of retinal ganglion cells (RGCs) and their axons, namely, the optic nerve, usually associated with elevated intraocular pressure (IOP). Current glaucoma therapies target reduction of IOP, but since RGC death is the cause of irreversible vision loss, neuroprotection may be an effective strategy for glaucoma treatment. One of the risk factors for glaucoma is increased oxidative stress, and drugs with antioxidative properties including valproic acid and spermidine, as well as inhibition of apoptosis signal-regulating kinase 1, an enzyme that is involved in oxidative stress, have been reported to prevent glaucomatous retinal degeneration in mouse models of glaucoma. Optic neuritis is a demyelinating inflammation of the optic nerve that presents with visual impairment and it is commonly associated with multiple sclerosis, a chronic demyelinating disease of the central nervous system. Although steroids are commonly used for treatment of optic neuritis, reduction of oxidative stress by approaches such as gene therapy is effective in ameliorating optic nerve demyelination in preclinical studies. In this review, we discuss oxidative stress as a therapeutic target for glaucoma and optic neuritis.

Spermidine acts as an endogenous free radical scavenger and inhibits the action of reactive oxygen species. In this study, we examined the effects of spermidine on retinal ganglion cell (RGC) death in a mouse model of optic nerve injury (ONI). Daily ingestion of spermidine reduced RGC death following ONI and sequential in vivo retinal imaging revealed that spermidine effectively prevented retinal degeneration. Apoptosis signal-regulating kinase-1 (ASK1) is an evolutionarily conserved mitogen-activated protein kinase kinase kinase and has an important role in ONI-induced RGC apoptosis. We demonstrated that spermidine suppresses ONI-induced activation of the ASK1-p38 mitogen-activated protein kinase pathway. Moreover, production of chemokines important for microglia recruitment was decreased with spermidine treatment and, consequently, accumulation of retinal microglia is reduced. In addition, the ONI-induced expression of inducible nitric oxide synthase in the retina was inhibited with spermidine treatment, particularly in microglia. Furthermore, daily spermidine intake enhanced optic nerve regeneration in vivo. Our findings indicate that spermidine stimulates neuroprotection as well as neuroregeneration, and may be useful for treatment of various neurodegenerative diseases including glaucoma.

PURPOSE. To assess the therapeutic potential of spermidine in mice with excitatory amino acid carrier 1 (EAAC1) deletion (EAAC1 knockout [KO] mice), a mouse model of normal tension glaucoma.METHODS. Spermidine, at 30 mM in drinking water, was administered to EAAC1 KO mice from 5 to 12 weeks old. Optical coherence tomography, multifocal electroretinograms, and the measurement of intraocular pressure (IOP) were performed at 5, 8, and 12 weeks old. Histopathology analyses were carried out at 8 and 12 weeks old, and immunoblot and immunohistochemical analyses of 4-hydroxy-2-nonenal (4-HNE) in the retina were performed at 8 weeks old. RESULTS. Spermidine ameliorated retinal degeneration and improved visual function in EAAC1KO mice at both 8 and 12 weeks old, without affecting IOP. A significant increase of 4-HNE was observed in vehicle-treated EAAC1 KO mice, but spermidine treatment reduced this increase, suggesting that spermidine alleviated the severity of the glaucoma-like phenotype by acting as an antioxidant. CONCLUSIONS.The results from this study suggest that oral spermidine administration could be a useful treatment for retinal degenerative disorders including glaucoma.