Changes in Redox Balance and Mitochondrial Activity in the Retinas of Cystine/Glutamate Antiporter Knockout Mice

The cystine/glutamate antiporter (xCT) mediates glutamate export and cyst(e)ine import. In the retina, we previously demonstrated that xCT is important in glutamate/glutamine cycling between photoreceptor and Müller cells. This study investigates the contribution of xCT to cyst(e)ine import and glutathione homeostasis and its impact on mitochondrial function.

C57BL/6J wild type (WT) and xCT knockout (KO) retinas were analyzed at six weeks and nine months. Mass spectrometry and silver-intensified immunogold labeling were used to measure cysteine (CSH) and glutathione (GSH) in the retinal layers, whereas high-resolution respirometry measured mitochondrial activity and reactive oxygen species (ROS) levels.

While CSH and GSH were similar between WT and KO whole retinas at both ages, localized reduction of CSH and GSH were evident in the photoreceptors. ROS levels increased in six-week KO compared to WT retinas, and these levels were sustained at nine months. At six weeks, but not nine months, loss of xCT resulted in increased mitochondrial complex I activity and reduced mitochondrial ROS levels.

As early as six weeks of age, the loss of xCT resulted in localized changes in CSH and GSH levels, suggesting that xCT plays a role in GSH homeostasis. An increase in overall ROS levels was detected, but this was not attributed to the mitochondria. Changes detected in six-week KO retinas were comparable to those seen in nine-month WT retinas, suggesting that loss of xCT may accelerate changes associated with aging. The lack of differences between WT and xCT KO retinas at nine months indicate adaptations to these early changes over time.