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Julie Lim, Leo Lam, Lincoln Peung, Bo Li, Paul Donaldson; Cysteine delivery to the lens: a strategy for delaying nuclear cataract?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2978.
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© ARVO (1962-2015); The Authors (2016-present)
To determine whether a cystine uptake pathway mediated by the cystine/glutamate exchanger (Xc-) and members of the glutamate transporter family (EAAT4 and ASCT2) shown to be expressed in the rat lens are also present in the human lens and to ascertain whether accumulation of cysteine by this pathway delays the formation of nuclear cataract.
RT-PCR and Western blotting were used to determine Xc-, EAAT4 and ASCT2 expression at the transcript and protein levels from human donor lenses. Immunohistochemistry was used to localise transporters to different regions of the lens and to monitor changes in transporter expression with increasing age. Bovine lenses were pre-incubated in the absence or presence of cysteine and exposed to hyperbaric oxygen (HBO) for 5 hours (100% O2, 100 atm). Biochemical parameters including glutathione and cysteine levels, lipid peroxidation (MDA), protein solubility and the formation of protein mixed disulfides were used to assess the effectiveness of cysteine supplementation.
Xc-, EAAT4 and ASCT2 were shown to be expressed in all regions of younger lenses. However, in older lenses, Xc- was detected only in the outer cortex. The absence of Xc- in deeper lying fiber cells corresponds to lack of cystine labelling in these same regions and indicates that while EAAT4 is present in nuclear regions, cystine uptake mediated by Xc-/EAAT4 plays a minimal contribution to overall cysteine accumulation in older lenses. In contrast, in older lenses ASCT2 was expressed in all regions suggesting that ASCT2 could be targeted to mediate direct cysteine uptake. Bovine lenses pre-incubated in the absence of cysteine and then exposed to HBO showed reduced levels of GSH, increased levels of MDA, decreased protein solubility and increased formation of protein mixed disulfides.
The presence of Xc- and EAAT4 throughout young lenses indicates that these transporters may work together to accumulate cystine. However, in older lenses, post translational modifications to Xc- may indicate that the lens centre is not able to effectively accumulate cystine. The presence of ASCT2 in the nucleus of older lenses indicates that cysteine can be accumulated directly. Further work using our HBO system and supplementation of bovine lenses with exogenous cysteine will determine if this is effective in delaying the biochemical changes associated with age related nuclear cataract.
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