The xCT knockout mouse: A novel animal model for studying age-related changes in the retina

Abstract

With an ageing population, the prevalence of retinal diseases including age-related macular degeneration is increasing. Since metabolic dysfunction and oxidative stress contribute to ageing, early interventions that target metabolic and redox balance may delay disease onset. The cystine/glutamate antiporter has been identified as a candidate to modulate oxidative stress through its role in intracellular glutathione production and extracellular cysteine/cystine redox balance, and in influencing energetic metabolism through glutamate export. To investigate the role of the cystine/glutamate antiporter in the eye, my laboratory established a global knockout (KO) mouse lacking the functional subunit xCT. In vivo examinations revealed accelerated formation of age-related deposits in xCT KO retinas compared to wild type (WT) retinas. Moreover, the ocular humours of xCT KO mice exhibited an oxidative shift in the cysteine/cystine ratio, indicative of accelerated ageing. In this thesis, I utilised the xCT KO mouse to investigate the metabolic, oxidative, and functional changes in the retina after loss of xCT to determine its role in the retina and evaluate whether this represents a model of retinal ageing. In this thesis, I discovered that removal of xCT caused glutamate accumulation in the photoreceptors, which was associated with altered mitochondrial function and reduced mitochondrial reactive oxygen species (ROS) production. Oxidative stress was heightened in xCT KO retinas, with increased ROS levels and depletion of glutathione in the photoreceptors. These changes occurred at 6-weeks of age in xCT KO retinas and were absent in age-matched WT retinas. I also characterised the retinal deposits which most likely represent photoreceptor outer segment debris which were associated with microglia/macrophages in the subretinal space. Focal electroretinography revealed that these deposits reduced the retinal response to light in their local area, but overall did not affect retinal function. Taken together, these results suggest that the xCT KO mouse exhibits altered metabolism, mitochondrial function, and ROS production characteristic of ageing, which precede the appearance of age-related deposits. This suggests that the xCT KO mouse is a useful model for studying the ageing retina which in the future could be used to test intervention strategies aimed at restoring metabolic and redox homeostasis and delaying the onset of age-related retinal diseases, for which no effective treatments currently exist.