The role of the cystine/glutamate antiporter in mouse ocular tissues

Abstract

With an aging population, the prevalence of age-related eye diseases such as cataract and age-related macular degeneration is increasing. Since oxidative stress is known to contribute to the pathogenesis of age-related eye diseases, strategies for minimising oxidative stress have been identified as a means of delaying the onset of these diseases. The cystine/glutamate antiporter has emerged as a promising candidate in other tissues to minimise oxidative stress in its ability to provide cystine for the intracellular synthesis of the antioxidant glutathione (GSH) and in the maintenance of extracellular cysteine/cystine redox balance. To investigate the roles of the cystine/glutamate antiporter in the eye, I have used a global knockout (KO) mouse that lacks xCT, the functional subunit of the antiporter. In this thesis, I demonstrate that loss of xCT results in extracellular redox imbalance of the plasma, aqueous, and vitreous humour. While cysteine levels were maintained, cystine levels were significantly increased in the KO compared to the wild-type (WT) mouse. This resulted in an oxidative shift of the cystine/cysteine ratio indicative of advanced aging. In support of this, in vivo examinations revealed KO mice developed age-related pathologies in the lens and retina earlier than WT mice. Interestingly, the absence of xCT did not affect intracellular GSH levels in any tissue suggesting that xCT may be important, but not essential for overall GSH homeostasis. Markers of oxidative damage were similar between WT and KO in the cornea but increased oxidative stress markers were seen in the young (6 weeks) to middle aged (12 months) KO lens and young KO retinas. Electroretinography revealed that loss of xCT results in delays in the photoreceptor and post-photoreceptor response time in the young KO retina. These delays were similar to that in older (9 months) WT retinas, suggesting that young KO retinas exhibit signs of a decline in retinal function more characteristic of an older retina. Taken together, these results suggest that the xCT KO mice is a useful model for study the aging eye and in the future could be used to test intervention strategies aimed at restoring cystine/cysteine redox homeostasis and delaying the onset of age-related eye diseases, for which currently no preventive treatment exists.