Selenite-induced epithelial damage and cortical cataract

Different experimental approaches in modelling cataractogenesis: An overview of selenite-induced nuclear cataract in rats

Cataract, the opacification of eye lens, is the leading cause of blindness worldwide. At present, the only remedy is surgical removal of the cataractous lens and substitution with a lens made of synthetic polymers. However, besides significant costs of operation and possible complications, an artificial lens just does not have the overall optical qualities of a normal one. Hence it remains a significant public health problem, and biochemical solutions or pharmacological interventions that will maintain the transparency of the lens are highly required. Naturally, there is a persistent demand for suitable biological models. The ocular lens would appear to be an ideal organ for maintaining culture conditions because of lacking blood vessels and nerves. The lens in vivo obtains its nutrients and eliminates waste products via diffusion with the surrounding fluids. Lens opacification observed in vivo can be mimicked in vitro by addition of the cataractogenic agent sodium selenite (Na₂SeO₃) to the culture medium. Moreover, since an overdose of sodium selenite induces also cataract in young rats, it became an extremely rapid and convenient model of nuclear cataract in vivo. The main focus of this review will be on selenium (Se) and its salt sodium selenite, their toxicological characteristics and safety data in relevance of modelling cataractogenesis, either under in vivo or in vitro conditions. The studies revealing the mechanisms of lens opacification induced by selenite are highlighted, the representatives from screening for potential anti-cataract agents are listed.

Regulatory effect of acetyl-l-carnitine on expression of lenticular antioxidant and apoptotic genes in selenite-induced cataract

Differential expression of apoptotic genes has been demonstrated in selenite-induced cataract. Acetyl-l-carnitine (ALCAR) has been shown to prevent selenite cataractogenesis by maintaining lenticular antioxidant enzyme and redox system components at near normal levels and also by inhibiting lenticular calpain activity. The aim of the present experiment was to investigate the possibility that ALCAR also prevents selenite-induced cataractogenesis by regulating the expression of antioxidant (catalase) and apoptotic [caspase-3, early growth response protein-1 (EGR-1) and cytochrome c oxidase subunit I (COX-I)] genes. The experiment was conducted on 9-day-old Wistar rat pups, which were divided into normal, cataract-untreated and cataract-treated groups. Putative changes in gene expression in whole lenses removed from the rats were determined by measuring mRNA transcript levels of the four genes by RT-PCR analysis, using glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as an internal control. The expression of lenticular caspase-3 and EGR-1 genes appeared to be upregulated, as inferred by detecting increased mRNA transcript levels, while that of COX-I and catalase genes appeared to be downregulated (lowered mRNA transcript levels) in the lenses of cataract-untreated rats. However, in rats treated with ALCAR, the lenticular mRNA transcript levels were maintained at near normal (control) levels. These results suggest that ALCAR may prevent selenite-induced cataractogenesis by preventing abnormal expression of lenticular genes governing apoptosis.

Sequential effects of ultraviolet radiation on the histomorphology, cell density and antioxidative status of the lens epithelium--an in vivo study

In vivo progressive effects of UV irradiation on the lens epithelium were studied using various histomorphological and biochemical parameters. Fifteen day old rat pups were exposed to 600 mW/m2 of radiation, including UV-A and UV-B, 12 h daily for 90, 120, 150 and 180 days. Biochemical parameters such as protein-bound and non-protein-bound sulfhydryl groups in both soluble and insoluble fractions and enzymes, which play an important role in combating the oxidative stress, were studied. Decreased cell density of lens epithelial cells (LEC) was observed in all three zones along with the decrease in the levels of soluble sulfhydryls (S-SH), glutathione reductase (GR), superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT). On the other hand, an increase in insoluble sulfhydryls was observed. Because of the decrease in S-SH and GR activities, the LEC became vulnerable to oxidative stress. Decreased activities of SOD, GPx and CAT suggest elevated oxidative stress. This effect of UV radiation may lead to cell death that may be responsible for the observed decrease in the cell density in all three zones of the lens epithelium.

Expression changes in mRNAs and mitochondrial damage in lens epithelial cells with selenite

An overdose of sodium selenite induces cataracts in young rats. The mid-stage events producing the cataract include calpain-induced hydrolysis and precipitation of lens proteins. Apoptosis in lens epithelial cells has been suggested as an initial event in selenite cataracts. Expression levels of two genes associated with apoptosis were altered in lens epithelial cells from selenite-injected rats. The purpose of the present experiment was to perform a more comprehensive search for changes in expression of mRNAs in lens epithelial cells in order to more fully delineate the early events in selenite-induced cataracts. Lens epithelial cells were harvested at 1 and 2 days after a single subcutaneous injection of sodium selenite (30 mumol/kg body weight) into 12-day-old rats. Gene expression was analyzed using a commercial DNA array (Rat Genome U34A GeneChip array, Affymetrix). Of approximately 8000 genes assayed by hybridization, 13 genes were decreased and 27 genes were increased in the rat lens epithelial cells after injection of selenite. Some of the up-regulated genes included apoptosis-related genes, and a majority of the down-regulated genes were mitochondrial genes. Previously observed changes in expression of EGR-1 mRNA were also confirmed. Changes in the expression patterns of mRNAs were also confirmed by RT-PCR. To determine the mechanism for damage of lens epithelial cells (alpha TN4 cell) by culture in selenite, leakage of cytochrome c from mitochondria was measured. Selenite caused significant leakage of cytochrome c into the cytosol of alpha TN4 cells. Our data suggested that the loss of integrity of lens epithelial cells by selenite might be caused by preferential down-regulation of mitochondrial RNAs, release of cytochrome c, and impaired mitochondrial function. Up-regulation of mRNAs involved in maintenance of DNA, regulation of metabolism, and induction of apoptosis may also play roles.

Decreased chaperone activity of alpha-crystallin in selenite cataract may result from selenite-induced aggregation

PURPOSE

To investigate the role of chaperone activity of alpha-crystallin in selenite-induced cataract formation.

METHODS

Selenite cataract was induced in Sprague-Dawley rats by five subcutaneous injections of sodium selenite over a 20-day period starting at 8-10 days postpartum. alpha-Crystallin was separated from the rat lenses by size-exclusion chromatography. Bovine alpha(L)-crystallin and beta(L)-crystallin were isolated for studies in vitro, and for the chaperone assays. The protective effects of both alpha(H)- and alpha(L)-crystallin were measured spectrophotometrically in four different assay procedures including the thermally induced aggregation of catalase and beta(L)-crystallin, and the fructation- and heat-induced inactivation of catalase. The bovine alpha(L)-crystallin was incubated with different concentrations of sodium selenite for 72 h and then its chaperone activity against heat-induced beta(L)-crystallin aggregation was assayed. The aggregation of selenite-treated alpha(L)-crystallin was analysed by molecular sieve high-performance liquid chromatography (HPLC).

RESULTS

The protection of alpha(H)-crystallin was less than that of alpha(L)-crystallin in both normal and cataractous lenses. The chaperone activities of both alpha(H)- and alpha(L)-crystallin in selenite cataract were decreased compared with normal lenses. The protection provided by both alpha(H)-crystallin and alpha(L)-crystallin against the thermal aggregation of catalase was much greater than their protection against thermally and chemically induced inactivation. HPLC analysis demonstrated aggregation of alpha-crystallin by sodium selenite after 24 h incubation in a dose-dependent fashion.

CONCLUSION

The chaperone activity of alpha-crystallin presented parallel patterns of activity with different methods, further supporting the view that the different assays measure essentially the same property. The decreased chaperone activity of alpha-crystallin in selenite cataract may result from selenite-induced aggregation.

Differential gene expression in the lens epithelial cells from selenite injected rats

The mechanism causing loss of integrity of lens epithelial cells induced by an overdose of sodium selenite remains to be elucidated. The aim of the present experiment was to search for changes in gene expression in epithelial cells of lenses from rats developing selenite cataract. One day after injection of selenite into 12 day old rats, gene expression in lens epithelial cells was analysed using a commercial DNA array (Atlas Rat 1.2 Array). Changes were confirmed by RT-PCR. Of 1176 genes assayed by hybridization, 91 genes showed differences in expression between normal and selenite lenses. The three genes showing the greatest changes were: cytochrome c oxidase subunit I (COX-I, decrease), gastric inhibitory polypeptide (GIP, decrease), and early growth response protein-1 (EGR-1, increase). Both COX-I and EGR-1 have been reported to be involved with apoptosis. These results suggest that changes in COX-I and EGR-1 expression in lens epithelial cells might play important roles in apoptosis and altered metabolism leading to selenite cataract.

Evidence for apoptosis in the selenite rat model of cataract

The purposes of this experiment were (1) to determine if apoptosis was accelerated during formation of selenite cataract, and (2) to determine the role of calpains and caspases in lens apoptosis. Evidence for apoptosis in selenite-injected rats included: approximately 7-8% of epithelial cells in germinative zone were positive, disappearance of the nuclear membrane, condensation of the chromatin, and breakdown of PARP. Activation of calpains was indicated by characteristic limited proteolysis of crystallins, breakdown of alpha-spectrin to 150/145 kDa fragments, hydrolysis of vimentin, and autolytic breakdown of m-calpain. Selenite cataract did not have an appreciable effect on the mRNA levels for caspase-3, calpains, and calpastatin. This indicated the increased enzyme activity of m-calpain and caspase-3 in selenite cataract occurred at the enzyme level rather than by upregulation of mRNAs. Increased calpain and caspase activity may be linked to the selenite-induced apoptosis. Such data are important because they indicate that apoptosis may be a fairly early event in selenite cataract.

Selenite nuclear cataractogenesis: a scanning electron microscope study

The sequential changes during selenite nuclear cataractogenesis were examined with a scanning electron microscope (SEM) and correlated with slit lamp observations. A posterior opacity, visible with the slit lamp 1-2 days after injection of sodium selenite, was found to consist of masses of vacuoles in the superficial posterior cortex by SEM. 2-3 days post injection, a biomicroscopic refractile ring around the nucleus was represented by SEM abnormalities suggesting membrane damage and possible loss of cytosol in the perinuclear region. All normal structure in this region was lost by 5 days after injection when the central nucleus had become opaque. SEM also showed evidence for damage in areas which were still clear by slit lamp examination. Changes, characteristic of aging, were found near selenite induced damage in peripheral (younger) fibers.

Review of selenite cataract

Recent advances in understanding the mechanism of selenite cataract have resulted from locating the cleavage sites on proteolyzed beta-crystallins from the cataract, mimicking the insolubilization of crystallins found in the cataract in an in vitro system, studying cataract produced in lenses cultured in selenite, and permanently or temporarily reducing the rate formation of selenite cataract by use of various inhibitors. The present review discusses the selenite cataract as a useful model for understanding the role calcium-induced proteolysis in cataract formation.

DNA damage, repair, and replication in selenite-induced cataract in rat lens

DNA synthesis was evaluated in vitro by measuring incorporation of 3H-thymidine in rat lens following systemic delivery of a cataractogenic dose of selenite. Among early metabolic changes observed in the lenses of rats receiving a single dose of 30 nmol Na2SeO3/g body weight was a 30% decrease in DNA replication in lens epithelium occurring between 6 and 12 h after administration of the selenite. This change was followed by an 80% increase in replication by 24 h. Thymidine incorporation in DNA remained elevated compared to controls through 96 h. Unscheduled DNA synthesis was found to be approximately 10% of the total DNA formed, but there was a 30% and 70% increase of this putative DNA repair in the lenses from selenite-treated animals at 6 and 24 h after the injection. Using the alkaline unwinding assay, the proportion of single-strand DNA in lenses from selenite-treated animals increased after 24 h. This estimate of DNA damage was greater in lenses after 96 h. Each component of DNA metabolism: damage, repair, and replication, was affected by the occurrence of selenite stress in lens. These changes both preceded and accompanied nuclear cataract formation.

Stationary radiation cataracts: an animal model

The restitution of normal fibergenesis that occurs in stationary radiation cataracts provides a unique opportunity to study the cytopathomechanism of radiocataractogenesis. Previous attempts at investigating this phenomenon have been limited by the lack of an appropriate animal model. This report describes the induction of stationary radiation cataracts in postmetamorphic bullfrogs following ocular irradiation with a 10 Gy (1 Gy = 100 rads) dose of X-rays. The eyes of non-irradiated animals and animals irradiated with 25 Gy (an established dose known to induce progressive cataracts in frogs) served as controls. Animals were followed biomicroscopically and histopathologically over 79 weeks. As previously described, the cataracts developed in a dose-dependent manner. The 25 Gy irradiated lenses rapidly progressed to complete opacification (4+) by 26 weeks, while lenses exposed to 10 Gy advanced to the 2.5+ stage by 35 weeks and progressed no further. In the lower dose lenses, transparent cortex began to appear anteriorly and posteriorly between the capsule and opaque fibers at 45 weeks. As the clear fibers accumulated, the disrupted region came to occupy increasingly deeper cortex. Histologically, opacities in both groups were preceded by disorganization of the bow cytoarchitecture, meridional row disorganization, and the appearance in the lens epithelium of nuclear polymorphism, fragmented nuclei, micronuclei, clusters of nuclei, and abnormal mitotic figures. In the lenses exposed to the 25 Gy dose, this damage continued to worsen, so that the 4+ stage was characterized by extensive epithelial cell death, absence of the lens bow, degenerated fiber masses, and liquefied substrata. In contrast, prior to the appearance of transparent cortex in the 10 Gy group, the lens epithelial aberrations, are of the bow, and meridional row disorganization were all observed to improve. Further, by 69 weeks, the lens epithelium appeared as a largely homogeneous population, and the meridional rows and the are of the bow had become reestablished. The details of these observations and their possible relationship to the cytopathomechanism of radiation cataract formation are discussed.

Direct chemical measurement of DNA synthesis and net rates of differentiation of rat lens epithelial cells in vivo: applied to the selenium cataract

This report describes a direct chemical method for rapidly estimating DNA synthesis and net rates of epithelial cell differentiation in the ocular lens in vivo. DNA synthesis in the lens of control and selenium-treated rats (12- or 13 days of age) was estimated by chemically isolating and measuring trichloroacetic acid (TCA)-insoluble 3H from the lens following injection of [3H]thymidine. Labeled substrate for DNA synthesis peaked in the lens at 1 hr after injection, decreased markedly by the third hour and was essentially gone by hour 12. Synthesis of labeled DNA in the lens was largely complete by about 3 hr. The [3H]DNA content of the whole lens, measured as TCA-insoluble 3H, remained constant for at least 4 months. The distribution of labeled epithelial cells between the epithelial-cell layer and fiber-cell mass was followed for up to 1 month after injection by measuring the ratio of [3H]DNA in the capsule (epithelial-cell layer) to lens body. Between days 2-3 and day 14 after injection, the ratio of [3H]DNA in the epithelial-cell layer to lens fiber cells decreased linearly in a semilogarithmic plot of the ratio vs. time; i.e. the rate of change of the ratio followed first-order kinetics. Thus, the rate constant (k) for the rate of change in the ratio of [3H]DNA in the capsule layer to lens body can provide an estimate of the percentage of the labeled epithelial cells which leave the capsule per day through differentiation into fiber cells. An apparent rate constant of 0.27 day-1 was estimated from the mean of five experiments; i.e. 27% of labeled epithelial cells were differentiating into cortical fiber cells per day. Therefore, about 70% of the germinative epithelial cells would be replaced every 4 days in these rats. This value is in good agreement with results of studies using autoradiographic technics. The selenium cataract is reported to involve rapid damage to lens epithelial cells. Incorporation of [3H]thymidine into DNA was decreased by at least 60% in the lens of selenium-treated rats. Selenium did not decrease the availability of substrate in the lens for DNA synthesis. The cause of the decreased incorporation of [3H]thymidine into the selenium-exposed lenses remains to be determined.

Selenite cataract: a review

Selenite cataract is a fairly recently described, experimental animal model for cataract (1). Selenite cataract has been extensively characterized histologically (2) and biochemically (3,4). The model has been particularly useful for studies on the roles of calcium accumulation and lens proteolysis in cataract formation (4). This review describes current knowledge of the biochemical mechanism for selenite cataract and indicates how the model may be used for further understanding of cataractogenesis in general.

Inhibition of Na,K-ATPase by sodium selenite and reversal by glutathione

Sodium selenite has been shown to inhibit Na,K-ATPase. Glutathione, at sufficient excess, is able to prevent or reverse the inhibition. Dithiothreitol can also reverse much of the inhibition, but KCN cannot. Selenomethionine does not inhibit Na,K-ATPase. The interactions of sodium selenite with Na,K-ATPase and glutathione may aid in understanding the early events in selenium cataractogenesis.