Mapping of the cystine-glutamate exchanger in the mouse eye: a role for xCT in controlling extracellular redox balance

Minimizing Oxidative Stress in the Lens: Alternative Measures for Elevating Glutathione in the Lens to Protect against Cataract

Oxidative stress plays a major role in the formation of the cataract that is the result of advancing age, diabetes or which follows vitrectomy surgery. Glutathione (GSH) is the principal antioxidant in the lens, and so supplementation with GSH would seem like an intuitive strategy to counteract oxidative stress there. However, the delivery of glutathione to the lens is fraught with difficulties, including the limited bioavailability of GSH caused by its rapid degradation, anatomical barriers of the anterior eye that result in insufficient delivery of GSH to the lens, and intracellular barriers within the lens that limit delivery of GSH to its different regions. Hence, more attention should be focused on alternative methods by which to enhance GSH levels in the lens. In this review, we focus on the following three strategies, which utilize the natural molecular machinery of the lens to enhance GSH and/or antioxidant potential in its different regions: the NRF2 pathway, which regulates the transcription of genes involved in GSH homeostasis; the use of lipid permeable cysteine-based analogues to increase the availability of cysteine for GSH synthesis; and the upregulation of the lens's internal microcirculation system, which is a circulating current of Na+ ions that drives water transport in the lens and with it the potential delivery of cysteine or GSH. The first two strategies have the potential to restore GSH levels in the epithelium and cortex, while the ability to harness the lens's internal microcirculation system offers the exciting potential to deliver and elevate antioxidant levels in its nucleus. This is an important distinction, as the damage phenotypes for age-related (nuclear) and diabetic (cortical) cataract indicate that antioxidant delivery must be targeted to different regions of the lens in order to alleviate oxidative stress. Given our increasing aging and diabetic populations it has become increasingly important to consider how the natural machinery of the lens can be utilized to restore GSH levels in its different regions and to afford protection from cataract.

Redox Regulation in Age-Related Cataracts: Roles for Glutathione, Vitamin C, and the NRF2 Signaling Pathway

Age is the biggest risk factor for cataracts, and aberrant oxidative modifications are correlated with age-related cataracts, suggesting that proper redox regulation is important for lens clarity. The lens has very high levels of antioxidants, including ascorbate and glutathione that aid in keeping the lens clear, at least in young animals and humans. We summarize current functional and genetic data supporting the hypothesis that impaired regulation of oxidative stress leads to redox dysregulation and cataract. We will focus on the essential endogenous antioxidant glutathione and the exogenous antioxidant vitamin C/ascorbate. Additionally, gene expression in response to oxidative stress is regulated in part by the transcription factor NRF2 (nuclear factor erythroid 2-related factor 2 [NFE2L2]), thus we will summarize our data regarding cataracts in Nrf2-/- mice. In this work, we discuss the function and integration of these capacities with the objective of maintaining lens clarity.

Early Onset of Age-Related Cataracts in Cystine/Glutamate Antiporter Knockout Mice

Purpose

The purpose of this study was to determine the importance of the xCT is a subunit. The cystine/glutamate antiporter is actually system xc-xCT subunit of the cystine/glutamate antiporter in maintaining redox balance by investigating the effects of the loss of xCT on lens transparency and cystine/cysteine balance in the aqueous humour.

Methods

C57Bl/6 wild-type and xCT knockout mice at five age groups (6 weeks to 12 months) were used. Lens transparency was examined using a slit-lamp and morphological changes visualized by immunolabelling and confocal microscopy. Quantification of glutathione in lenses and cysteine and cystine levels in the aqueous was conducted by liquid chromatography tandem mass spectrometry (LC-MS/MS).

Results

Slit-lamp examinations revealed that 3-month-old wild-type mice and xCT knockout mice lenses exhibited an anterior localized cataract. The frequency of this cataract significantly increased in the knockout mice compared to the wild-type mice. Morphological studies revealed a localized swelling of the lens fiber cells at the anterior pole. Glutathione levels in whole lenses were similar between wild-type and knockout mice. However, glutathione levels were significantly decreased at 3 months in the knockout mice in the lens epithelium compared to the wild-type mice. Aqueous cysteine levels remained similar between wild-type and knockout mice at all age groups, whereas cystine levels were significantly increased in 3-, 9-, and 12-month-old knockout mice compared to wild-type mice.

Conclusions

Loss of xCT resulted in the depletion of glutathione in the epithelium and an oxidative shift in the cysteine/cystine ratio of the aqueous. Together, these oxidative changes may contribute to the accelerated development of an anterior cataract in knockout mice, which appears to be a normal feature of aging in wild-type mice.

Gold nanoparticle synthesis in contact lenses for drug-less ocular cystinosis treatment

PURPOSE

To develop gold nanoparticles-loaded contact lens ("GoldinLens") to bind a significant mass of cystine on the surface of the gold nanoparticles (GNPs) for cystinosis treatment due to the reaction between cystine and gold.

METHODS

The GoldinLens was manufactured by synthesizing GNPs inside the preformed contact lens matrix by first loading the lenses (Moist and TrueEye) with gold precursor followed by reduction (with sodium borohydride or trisodium citrate) to gold atoms, which nucleated to GNPs inside the polymeric matrix. The lenses were characterized by SEM, XRD, UV-Vis spectroscopy and mass of GNPs loaded in the lens was determined by direct measurement of mass. Manufactured lenses were soaked in cystine solution for cystine uptake in vitro.

RESULTS

Results show that gold loading in the contact lens increases linearly with gold precursor concentration and number of repetitions of the manufacturing process. The stronger reducing agent sodium borohydride resulted in higher gold loading, with the loading being higher in the Moist lenses due to higher diffusivity of the reducing agent into the lens. However, GNPs were smaller in size and relatively monodispersed in TruEye GoldinLens, resulting in higher cystine uptake of 47 μg/lens over 24 h (vs. 33 μg/lens for Moist GoldinLens). However, the rate of this uptake was higher for Moist GoldiLens (8.25 vs. 2.35 μg/h), with the maximum uptake occurring in one hour (vs. five hours).

CONCLUSION

A method for manufacturing GoldinLens, wherein small gold nanoparticles are trapped in contact lenses, has been developed for drugless cystinosis treatment. The lenses withdraw cystine molecules from the surrounding milieu, with the TrueEye GoldinLens being superior for the extent of, while Moist GoldinLens is superior for rate of cystine removal. GoldinLenses of this study can be used for drugless cystine removal cystinosis treatment with one- or five-hour wear at a time.

Identification, Expression, and Roles of the Cystine/Glutamate Antiporter in Ocular Tissues

The cystine/glutamate antiporter (system x _c ^-) is composed of a heavy chain subunit 4F2hc linked by a disulphide bond to a light chain xCT, which exchanges extracellular cystine, the disulphide form of the amino acid cysteine, for intracellular glutamate. In vitro research in the brain, kidney, and liver have shown this antiporter to play a role in minimising oxidative stress by providing a source of intracellular cysteine for the synthesis of the antioxidant glutathione. In vivo studies using the xCT knockout mouse revealed that the plasma cystine/cysteine redox couple was tilted to a more oxidative state demonstrating system x_c ^- to also play a role in maintaining extracellular redox balance by driving a cystine/cysteine redox cycle. In addition, through import of cystine, system x_c ^- also serves to export glutamate into the extracellular space which may influence neurotransmission and glutamate signalling in neural tissues. While changes to system x_c ^- function has been linked to cancer and neurodegenerative disease, there is limited research on the roles of system x_c ^- in the different tissues of the eye, and links between the antiporter, aging, and ocular disease. Hence, this review seeks to consolidate research on system x_c ^- in the cornea, lens, retina, and ocular humours conducted across several species to shed light on the in vitro and in vivo roles of xCT in the eye and highlight the utility of the xCT knockout mouse as a tool to investigate the contribution of xCT to age-related ocular diseases.

Impact of sex, age and diet on the cysteine/cystine and glutathione/glutathione disulfide plasma redox couples in mice

Age, sex and diet are well-established risk factors for several diseases. In humans, each of these variables has been linked to differences in plasma redox potentials (E_h) of the glutathione/glutathione disulfide (GSH/GSSG) and cysteine/cystine (Cys/CySS) redox couples. Mice have been very useful for modeling human disease processes, but it is unknown if age, sex and diet affect redox couples in mice as they do in humans. The purpose of the present study was to examine the effects of these factors on plasma redox potentials in C57BL/6J mice. We found that age had no effect on either redox couple in either sex. Plasma E_h Cys/CySS and E_h GSH/GSSG were both more oxidized (more positive) in females than in males. A 24-hour fast negated the sex differences in both redox potentials by oxidizing both redox couples in male mice, while having no effect on E_h Cys/CySS and a smaller effect on E_h GSH/GSSG in female mice. A diet with excess sulfur amino acids reduced the plasma E_h Cys/CySS in females to a level comparable to that seen in male mice. Thus, sex-specific differences in plasma E_h Cys/CySS could be normalized by two different dietary interventions. Some of these findings are consistent with reported human studies, while others are not. Most strikingly, mice do not exhibit age-dependent oxidation of plasma redox potentials. Care must be taken when designing and interpreting mouse studies to investigate redox regulation in humans.