Calcium cataract: a model for optical anisotropy fluctuations

Alterations in the lenticular protein profile in experimental selenite-induced cataractogenesis and prevention by ellagic acid

BACKGROUND

Accumulating evidence suggests that oxidative stress underlies age-related formation of cataract, and that antioxidants retard cataractogenesis. This study aimed to evaluate whether ellagic acid, a natural polyphenol with antioxidant properties, prevents alterations in the lenticular protein profile in an experimental model of selenite cataract.

METHODS

Alterations in lenticular protein were determined by two-dimensional electrophoresis (2DE) and image analysis. Eluted αA-crystallin spots were analyzed by mass spectrometry. Western blot analysis was also performed to confirm the differential expression of certain crystallins and cytoskeletal proteins.

RESULTS

In cataractous lenses, 2DE and image analysis revealed approximately 45 and 60 prominent spots in soluble and insoluble protein fractions respectively. Analysis of the pI and molecular weight of protein spots revealed differences in the expression of crystallin proteins in soluble and insoluble fractions. Western blot analysis confirmed changes in the expression of αA- and βB1- crystallins in both soluble and insoluble protein fractions, while mass spectrometry confirmed the degradation of αA-crystallin in selenite cataractous lenses. Western blot analysis also confirmed the occurrence of altered expression of certain cytoskeletal proteins in insoluble fractions. However, the lenticular protein profile in lenses from selenite-challenged, ellagic acid-treated rats was essentially similar to that noted in lenses from normal rats.

CONCLUSIONS

The present study confirms the importance of structural and cytoskeletal proteins in the maintenance of lenticular transparency; the results also suggest that ellagic acid prevents lenticular protein alterations induced by selenite in an experimental setting.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

Lens intermediate filaments

The ocular lens assembles two separate intermediate filament systems sequentially with differentiation. Canonical 8-11 nm IFs composed of Vimentin are assembled in lens epithelial cells and younger fiber cells, while the fiber cell-specific beaded filaments are switched on as fiber cell elongation initiates. Some of the key features of both filament systems are reviewed.

Determinations of some trace and heavy metals in rat lenses after tobacco smoke exposure and their relationships to lens injury

Cigarette smoking has been implicated in the pathogenesis of cataract, but the pathogenic mechanism by which cigarette smoke causes cataract is yet to be completely understood. There has been suggestion that oxidative damage caused by accumulation of Fenton reagents (iron and copper) in the lens can cause lens damage and possibly cataract. To investigate the accuracy of this theory the study was planned. A number of twenty-four male Wistar rats were divided randomly into experimental and control groups. The experimental group of rats were exposed to cigarette smoke for two hours in each day over sixty consecutive days and the controls were treated in identical fashion but only exposed to room air. At the end of the study period, both eyes of all the animals were enucleated and one eye prepared for histopathological examination and the other used for the measurement of metal levels. The lenses of experimental animals showed significantly decreased zinc and increased iron, and calcium concentration relative to those of sham exposed controls. However, no significant difference was found in the copper contents of the lenses of both groups. Distinct histopathological changes such as hyperplasia, hypertrophia, and multilayering of epithelial cells and elevations of calcium concentration detected in the lenses of experimental group animals suggested that the lens damage was a result of in-vivo exposure to tobacco smoke. We propose that increased metal contents in the lens can cause lens damage by the mechanism of oxidative stress through formation of oxygen radicals via metal catalysed Fenton reaction.