Nutrient Transporters In The Normal And Cataractic Lens

Age-Related Nuclear (ARN) Cataract - a Transport Problem?

As the world's population ages, ARN cataract has become the leading cause of blindness. It is characterized by a drastic decline in GSH concentration in the nucleus of the lens, but not the cortex, exposing the centre of the lens to the damaging effects of oxygen radicals, causing protein aggregation, increased light scattering and ultimately nuclear cataract (65). However, unlike diabetic cataract, in ARN cataract there appears to no significant morphological changes to the cellular architecture of the lens. Biochemically, however, extensive modifications to proteins in the lens nucleus are observed that include oxidation of methionine residues and sulfhydryl groups, insolubilization of crys-tallins, and protein cross-linking to form mixed disulphides, all of which contribute to light scattering (66).

Oxidative damage is a key feature of ARN cataracts. In the transparent lens, a robust oxygen-radical scavenger system which utilizes glutathione (GSH) as its principal antioxidant, guards against oxidative stress (67). In the lens, GSH exists in unusually high concentrations (around 10 mM). These high levels are established via the direct uptake of GSH from the aqueous humor(68-70) and/or the endogenous synthesis of GSH from its precursor amino acids cysteine, glutamate, and glycine (71) by the sequential actions of the enzymes y-glutamylcysteine synthetase (yGCS) and glutathione synthetase (GS) in the lens cortex. GSH levels are then maintained by the regeneration of GSH from the oxidized form of GSH (GSSG) by the enzyme glutathione reductase (GR), and the consumption of reducing equivalents such as NADPH (72).

In ARN cataract, the levels of GSH are abruptly reduced in the nucleus relative to the cortex, making the centre of the lens especially susceptible to oxidative damage (65, 73). Since the levels of GSH and the activities of its associated enzymes have been shown to progressively decline as a function of age, it has been assumed that ARN cataract is the result of a failure of enzymatic activity (74). However, while the specific activities of enzymes were reduced with increasing age, these reductions were not only deemed to be insufficient to account for the decrease in GSH levels observed in the nucleus, but they do not explain the abrupt fall in GSH levels seen in ARN cataract (75). Rather, nutrient transport to the lens nucleus may be an underlying factor in the development of ARN cataract (76). This raises the question of whether the fall in GSH in the lens nucleus observed during ARN cataract progression is in fact a transport problem.

Differential Expression of Nutrient Transporters in the Lens

While mature fibre cells in the lens nucleus are devoid of cellular organelles, they still require a supply of nutrients for anaerobicmetabolism,thereplenishmentofantioxi-dants and the maintenance of transparency. The traditional view is that the lens nucleus receives its nutrients from the cortex via an intercellularpathwaymediatedby gapjunc-tions. Indeed, Sweeney and Truscott have proposed that with advancing age, a barrier d evelops that restricts the intercellular diffusion of GSH from the cortex to the lens nucleus (76). The alternative view is that the circulation system causes convection of nutrients and antioxidants into the lens nucleus via an extracellular route faster than would be achieved by passive diffusion alone (6, 7). If this notion is correct, we would expect mature fibre cells to express transporters to accumulate the molecules delivered t o them by the circulation system. Consistent with this view, molecular studies ( able 1) have shown that fibre cells express a full repertoire of transporters that mediate the uptake of glucose (17, 32) and of the amino acids (15, 16, 77) involved in the synthesis of the GSH.

Furthermore, immunocytochemical localization of these transporters has shown that differences exist in the complement of transporters expressed in the cortex and nucleus of the lens, suggesting that regional differences in nutrient uptake may exist. Merriman-Smith et al. (17, 31, 32) have shown that while lens epithelial cells express the facilitative glucose transporter GLUT1, fibre cells express both the higher affinity GLUT3 isoform, and the sodium-dependent glucose transporter SGLT2 (Fig. 6A). Both the GLUT3 and SGLT2 transporters appear to be present predominantly in the cytoplasm of peripheral fibre cells and become inserted into the plasma membrane at different stages of fibre-cell differentiation. The GLUT3 transporter is initially inserted into the narrow sides of the differentiating fibre cells, while SGLT2 is abruptly inserted into the membranes of mature fibre cells at the transition zone between differentiating and mature fibre cells that coincides with the loss of cell nuclei. We have proposed that the observed differential expression of glucose transporters establishes an affinity gradient which increases the ability of deeper fibre cells to extract a diminishing supply of glucose from the extracellular space. In this regard, the expression of SGLT2 in the lens nucleus means that mature fibre cells are able to utilize the energy stored in the sodium gradient to accumulate glucose above its concentration gradient, where it can be used for anaerobic metabolism.

A similar change in the complement of transporters that mediate cystine uptake in the cortex and nucleus has also been observed in the rat lens (15, 16). Cystine, the dimeric oxidized form of cysteine, is more stable than cysteine, and is more abundant in the aqueous humor (69). Upon intracellular accumulation cystine is rapidly reduced

A: Glucose uptake

B: Cystine uptake pH 7.0 pH 6.5

Xc- EAAT4/5 Xc

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