Role of calpain in hydrogen peroxide induced cataract

Effect of hydrogen peroxide on lens transparency, intracellular pH, gap junction coupling, hydrostatic pressure and membrane water permeability

Clouding of the eye lens or cataract is an age-related anomaly that affects middle-aged humans. Exploration of the etiology points to a great extent to oxidative stress due to different forms of reactive oxygen species/metabolites such as Hydrogen peroxide (H2O2) that are generated due to intracellular metabolism and environmental factors like radiation. If accumulated and left unchecked, the imbalance between the production and degradation of H2O2 in the lens could lead to cataracts. Our objective was to explore ex vivo the effects of H2O2 on lens physiology. We investigated transparency, intracellular pH (pHi), intercellular gap junction coupling (GJC), hydrostatic pressure (HP) and membrane water permeability after subjecting two-month-old C57 wild-type (WT) mouse lenses for 3 h or 8 h in lens saline containing 50 μM H2O2; the results were compared with control lenses incubated in the saline without H2O2. There was a significant decrease in lens transparency in H2O2-treated lenses. In control lenses, pHi decreases from ∼7.34 in the surface fiber cells to 6.64 in the center. Experimental lenses exposed to H2O2 for 8 h showed a significant decrease in surface pH (from 7.34 to 6.86) and central pH (from 6.64 to 6.56), compared to the controls. There was a significant increase in GJC resistance in the differentiating (12-fold) and mature (1.4-fold) fiber cells compared to the control. Experimental lenses also showed a significant increase in HP which was ∼2-fold higher at the junction between the differentiating and mature fiber cells and ∼1.5-fold higher at the center compared to these locations in control lenses; HP at the surface was 0 mm Hg in either type lens. Fiber cell membrane water permeability significantly increased in H2O2-exposed lenses compared to controls. Our data demonstrate that elevated levels of lens intracellular H2O2 caused a decrease in intracellular pH and led to acidosis which most likely uncoupled GJs, and increased AQP0-dependent membrane water permeability causing a consequent rise in HP. We infer that an abnormal increase in intracellular H2O2 could induce acidosis, cause oxidative stress, alter lens microcirculation, and lead to the development of accelerated lens opacity and age-related cataracts.

Expression of calpain small subunit 2 in mammalian tissues

PURPOSE

The purpose of the current experiments was to more closely define the distribution and the function of calpain small subunit 2 (css2). Css2 is a newly discovered regulatory protein for the calcium activated proteases, mu- and m-calpains.

METHODS

Tissues from rat, monkey, and man of various ages were used to determine expression patterns of css2 by relative quantitative RT-PCR using 18S rRNA as an endogenous standard. Recombinant css2 and the 80 kDa catalytic subunit of m-calpain (80 kDa/css2) were co-expressed in Escherichia coli. Casein zymography was used to measure the enzymatic activity of 80 kDa/css2 proteins. Lens alpha-crystallin and beta B1-crystallin were used as substrates to determine proteolysis by 80 kDa/css2. Computer-based homology modeling was used to predict interactions between the traditional small subunit (css1) or css2 with the 80 kDa catalytic subunit.

RESULTS

Css2 appears to be a functional equivalent of css1 in vitro in that the calcium-dependent proteolytic activity of 80 kDa/css2 was similar to recombinant m-calpain (80 kDa/css1). In rat and human lens, css2 transcripts increased with age, whereas css1 transcripts decreased with age. Human beta B1-crystallin and rat alpha A-crystallin were cleaved similarly by 80 kDa/css2 and 80 kDa/css1. Interestingly, alpha A-insert crystallin was not hydrolyzed when css2 was substituted for css1 in the calpain dimer, suggesting that css2 may perform different functions from css1 in terms of proteolysis of lens crystallins during maturational growth of the lens. Css2 may also assist in the proper folding of the 80 kDa subunit and regulate protease activity in the absence of calcium.

CONCLUSIONS

The wide distribution of css2 transcripts in rat and monkey suggested that css2 is a second, widely distributed (rather than tissue-specific) calpain small subunit, in addition to the long-recognized css1. Further studies at the protein level will indicate if css2 has unique functions apart from css1.

Evaluation of a novel calpain inhibitor as a treatment for cataract

PURPOSE

The aim of this study is to evaluate the therapeutic potential of a newly synthesized calpain inhibitor, CAT0059, using a naturally occurring in vivo sheep cataract model.

METHODS

The selectivity of CAT0059 was investigated by an in vitro protease assay. The efficacy of CAT0059 in preventing proteolysis of lens cytoskeletal proteins by calpain 2 was investigated using a lens-based cell-free method. The cytotoxicity and stability of CAT0059 in physiological conditions were examined using cultured sheep lenses. Protein binding of CAT0059 by ocular proteins was assessed and quantified by a modified high-performance liquid chromatography assay. CAT0059 was formulated in an eye drop solution and as an eye ointment. These were applied in vivo daily to one eye of the cataract lambs, over a 67- and 97-day trial period, respectively. The progression of cataracts in the treated and untreated eyes was assessed by an independent veterinary ophthalmologist using a slit-lamp microscope.

RESULTS

In vitro assays revealed that CAT0059 was selective for cysteine proteases and also protected lens cytoskeletal proteins from degradation. CAT0059 was stable in physiological conditions and non-toxic to the lens. Only 15% of CAT0059 is bound to proteins in the aqueous humour but >90% bound to lens homogenate. The 67-day CAT0059 eye drop treatment was not effective in slowing the rate of cataract development. However, application of CAT0059 in an eye ointment initially slowed cataract development compared with the untreated eye. This effect was temporary.

CONCLUSIONS

In vitro assays confirmed CAT0059 to be a potent calpain inhibitor. The two in vivo trials addressed the ability of CAT0059 to reach the lens and established its limitations as a therapeutic molecule for cataract treatment.

Effect of oxidized βB3-crystallin peptide (152–166) on thermal aggregation of bovine lens γ-crystallins: identification of peptide interacting sites

We studied the effect of oxidized betaB3-crystallin peptide (residues 152-166) on the thermal aggregation of bovine gamma-crystallin and on chaperone activity of alpha-crystallin. Thermal aggregation of gamma-crystallin was higher in the presence of oxidized betaB3-crystallin peptide than without oxidized peptide. Increased aggregation was not observed in the presence of unoxidized betaB3-crystallin peptide or a control oxidized peptide. Enhanced aggregation of gamma-crystallin by oxidized betaB3-crystallin peptide was observed even at 37 degrees C. Interaction with oxidized betaB3-peptide increased the hydrophobicity in the gamma-crystallin as shown by increased 4, 4'-dianilino-1, 1'-binaphthyl-5, 5'-disulfonic acid (bis-ANS) binding. Enhanced aggregation of gamma-crystallin was observed despite the presence of alpha-crystallin (a chaperone protein) in the system. Sulfo succinimidyl-2-[6-(biotinamido)-2-{p-azidobenzamido}-hexanoamido]ethyl-1-3 dithio propionate (Sulfo-SBED) cross-linker studies further confirmed the interaction between oxidized betaB3-crystallin peptide and gamma-crystallin. Peptide interacted sites in gamma-crystallin were identified by matrix assisted laser desorption time-of-flight mass spectrometric methods and the result suggests that oxidized betaB3-crystallin peptide interacted with amino acid residues present on the outer surface of the gamma-crystallin. These results imply that oxidized betaB3-crystallin peptide interact with gamma-crystallins and enhance their aggregation and light scattering.

Identification and properties of anti-chaperone-like peptides derived from oxidized bovine lens betaL-crystallins

Thermal aggregation of betaL-crystallin was higher in the presence of peptide fragments generated from oxidized and trypsin-digested betaL-crystallin compared with thermal aggregation of the control proteins without oxidized betaL-crystallin fragments. Increased aggregation of betaL-crystallin was also observed despite the presence of alpha-crystallin (which has anti-aggregating properties) in the system. Self-aggregation of the oxidized betaL-crystallin fragments per se was not observed under the experimental conditions. Reverse-phase HPLC analysis of the precipitate obtained after heating a mixture of betaL-crystallin and oxidized betaL-crystallin fragments revealed that more than one peptide co-precipitates with betaL-crystallin. Electrospray mass spectrometry analysis of the peptides revealed that the molecular weight(s) of the peptides ranged from 1400-1800. Tandem mass spectrometry and a data base search revealed that two of the peptides originated from betaA4-crystallin (LTIFEQENFLGR, residues 121-132) and betaB3-crystallin (AINGTWVGYEFPGYR, residues 153-167) respectively. Oxidized synthetic peptides representing the same sequence were also found to enhance the aggregation of betaL-crystallin in a manner similar to oxidized lens betaL-crystallin peptides. These data suggest that the polypeptides generated after oxidation and proteolysis of betaL-crystallins interact with denaturing proteins and facilitate their aggregation and light scattering, thus behaving like anti-chaperones.

Can drugs or micronutrients prevent cataract?

Cataract is the major cause of blindness and of visual impairment worldwide, so its prevention is of the greatest importance. At present no drug therapy is licensed for use in the UK or the US, so the only treatment for cataract is by surgery, which is expensive and has adverse effects. This article reviews research on prevention of cataract by a variety of agents, including micronutrients as well as drugs. Benefits have been claimed for many compounds or mixtures and this review concentrates on those most extensively studied. Information on possible benefits of putative anticataract agents comes from a variety of approaches, from laboratory experiments, both in vitro and in vivo, to epidemiological studies in patients. Sorbitol-lowering drugs were the first to be examined systematically and progressed to clinical trials which were disappointing, and now the entire rationale for their use in prevention of cataract is questionable. Micronutrients showed little promise in animals but came to clinical trial in patients with cataract without the publication of any major benefit. Pantethine showed more promise in animal studies but the only clinical trial was abandoned early. A variety of laboratory and epidemiological evidence supports the benefits of aspirin-like drugs but there has been no trial specifically in patients with cataract. Add-on studies to trials of aspirin for other indications have not been encouraging. Research into other compounds is interesting but less advanced.

Calpain-induced light scattering in young rat lenses is enhanced by UV-B

The purpose of this study is to determine if UV-B enhances light scattering after proteolysis of crystallins by calpains, and to determine if lens-specific calpain Lp82 is involved, along with m-calpain, in the mechanism of in vitro precipitation. Lens soluble proteins from young rats were hydrolyzed for 24 hr by endogenous lens calpains, and the proteins were further incubated for up to 7 days with periodic irradiation by UV-B. Light scattering was measured daily at 405 nm. SDS-PAGE and immunoblotting assessed proteolysis of crystallins, activation of calpains, and formation of high molecular weight aggregations. Appreciable light scattering occurred in lens soluble proteins after proteolysis of crystallins by m-calpain and Lp82. UV-B markedly enhanced this light scattering and the formation of higher molecular weight aggregates consisting of proteolyzed alpha- and beta- and intact gamma-crystallins. Calpain inhibitor E64 and antioxidants DTE or GSH prevented the light scattering. These results show that calpain-induced light scattering is enhanced by the natural oxidant UV-B. Activation of Lp82, along with m-calpain, contributed to the light scattering. The linkage between proteolysis and oxidation is important because both oxidation and truncation of crystallins are found in aged human lenses, which are constantly exposed to UV irradiation.

Comparison of various calpain inhibitors in reduction of light scattering, protein precipitation and nuclear cataract in vitro

PURPOSE

To compare effects of calpain inhibitors on in vitro light-scattering in rat lens soluble protein and calcium-ionophore (A23187)-induced cataract formation in cultured rat lenses.

METHODS

Rat lens soluble protein was hydrolyzed for 24 hours by activation of endogenous lens calpain. Ten calpain inhibitors were tested in this model at 10 and 25 microM concentration. As an index of protein precipitation, light scattering was measured daily at 405 nm for 8 days. Lens proteins were analyzed by isoelectric-focussing. Subsequently, rat lenses were cultured for 5 days with 10 microM A23187. Calpain inhibitors (SJA6017, MDL28170, AK295 and PD150606), which inhibited light-scattering were tested at 100 microM concentration in this model. Cataract evaluation, isoelectric-focussing and calcium determinations were performed.

RESULTS

At 25 microM concentration AK295, SJA6017, E-64, PD-150606 and MDL28170 produced greater than 25% inhibition of light-scattering. Isoelectric-focussing revealed that addition of Ca(2+) produced characteristic crystallin proteolysis and aggregation patterns. AK295, SJA6017, MDL28170 and E64c prevented these changes. Lenses cultured in A23187 exhibited nuclear cataract, elevated calcium and proteolysis and aggregation of crystallins. Co-culture with SJA6017, MDL28170 and E64c reduced A23187-induced nuclear opacities, proteolysis and aggregation of crystallins without affecting increased total calcium.

CONCLUSIONS

Endogenous calpain-activation model and A23187-induced cataract model can be used sequentially to screen calpain inhibitors for potential anti-cataract activity. Proteolytic changes in lens cortex after exposure to A23187 are also due to calpain activation. AK295, SJA6017 and MDL28170 possess efficacy against calcium-induced models of rodent cataracts. Use of calpain inhibitors represents a promising approach to cataract therapy.

The relationship between osmotic stress and calcium elevation: in vitro and in vivo rat lens models

Both in vivo and in vitro models were employed in the present study to assess the relative contribution of osmotic stress and increasing calcium levels to the development of sugar cataracts. In galactose cataract obtained from galactosemic weanling rats, the concentration of total calcium increased by nearly 10% at the first sign of visible opacification observed on the fourth day post-galactose feeding. After 7 days of galactose feeding, calcium levels continued to rise, to 0.8 mM. During the first 10 days, loss of lens transparency and calcium elevation was gradual and steady, with precipitous changes occurring on days 11 and 12. In groups of rats where galactose feeding was stopped after 7 days, cataract reversal was followed during the next 5 weeks. During the initial first week of recovery, calcium influx and elevation in the lens continued but began to decline steadily thereafter. After 3 weeks of recovery, lens transparency had returned to almost normal. Calcium levels continued to decline and reached normal levels between day 34 and 42, nearly 4 weeks after removal of the galactose diet. The relationship between osmotic stress and calcium elevation was investigated more directly by culturing normal rat lenses in hypoosmotic medium (280 mOsm) to create osmotic gradients similar to that in galactosemic lenses. The results showed that during the first day of culture (12 hr), osmotically stressed lenses gained 3 mg of water, became opaque and gained excess calcium (7 mM compared to 0.7 mM). Microscopic vacuoles appeared to accompany the process of opacification and contributed to increased light scattering and the loss of lens transparency. Additional experiments were designed to further distinguish between the effects of osmotic stress and calcium elevation on the opacification process. Thus, lenses were incubated in control and high-calcium medium (20 mM) at 300 mOsm. Within 12 hr of incubation, calcium elevation progressed to 1.37 mM, nearly doubling the normal value. Although opacification was observed in these lenses, no sign of vacuoles was evident. Collectively, the findings from this study support the premise that an early influx of calcium is brought about by osmotic stress and is responsible for the observed loss in transparency in osmotic (sugar) cataract.

Evidence for the involvement of calpain in cataractogenesis in Shumiya cataract rat (SCR)

The Shumiya cataract rat (SCR) is a hereditary cataract model in which lens opacity appears spontaneously in the nuclear and perinuclear portions at 11-12 weeks of age. It was found that the proteolysis of some crystallins and cytoskeletal proteins is significantly enhanced in cataractous SCR lenses. The calcium concentrations in cataractous lenses rise markedly with age as compared with control lenses and the autolytic product of calpain is also detected in cataractous lenses. In order to provide direct evidence for the involvement of calpain in the proteolytic modification of lens proteins, we developed antibodies exclusively specific to the proteolytic products of some lens proteins produced by the action of calpain and analyzed their degradation during cataractogenesis in SCR by Western blotting and immunohistochemical staining. The results demonstrate that calpain participates in the proteolytic modification of lens proteins, at least alpha-crystallin (A and B chain), betaB1-crystallin, and alpha-fodrin. The proteolytic products formed by the action of calpain on these proteins are detected in cataractous lenses of SCR as young as 8 weeks of age and accumulate with age. It was also found that betaB1-crystallin, originally a soluble protein, is converted to an insoluble form by limited calpain proteolysis. The chaperon-like activity of alpha-crystallin from control lens is markedly reduced by calpain proteolysis in vitro, and alpha-crystallin in opaque lens that has already undergone proteolysis by calpain shows significantly reduced chaperon-like activity. Immunohistochemical studies reveal that the area where the calpain-mediated alpha-crystallin proteolysis is in progress coincides well with the area developing and destined to develop the opacification. These results strongly suggest that calpain may contribute to lens opacification during cataract formation in SCR.

Mechanistic analysis of S-(1,2-dichlorovinyl)-L-cysteine-induced cataractogenesis in vitro

Chronic exposure to low concentrations of the nephrotoxic cysteine conjugate S-(1,2-dichlorovinyl)-l-cysteine (DCVC) causes cataracts in mice. This study explored mechanisms of DCVC-induced cataractogenesis using explanted lenses from male Sprague-Dawley rats. Lenses placed in organ culture were exposed to 2.5 microM-1 mM DCVC for 24 hr. DCVC caused concentration and time-dependent changes in biochemical markers of toxicity (lenticular adenosine 5'-triphosphate (ATP) content, mitochondrial reduction of the tetrazolium dye MTT, and glutathione (GSH) content) at concentrations >/=25 microM. Lens clarity was adversely affected at concentrations >/=50 microM. Within 24 hr, 1 mM DCVC altered lens ATP content (-77 +/- 2%), mitochondrial MTT reduction (-40 +/- 3%), and GSH content (-19 +/- 4%) (percent difference from controls, p < 0.05). ATP was the most sensitive index of DCVC exposure in this model, while lens weight was not altered. The role of lenticular DCVC metabolism was investigated using the beta-lyase inhibitor aminooxyacetic acid (AOA) and the flavin monooxygenase (FMO) inhibitor methimazole (MAZ). AOA (1 mM) provided nearly complete protection from changes in biochemical parameters and lens transparency caused by DCVC, while MAZ (1 mM) provided only partial protection. The mitochondrial Ca2+ uniport inhibitor ruthenium red (30 microM) and the poly(ADP ribosyl)transferase inhibitor 3-aminobenzamide (3 mM) were only partially protective, whereas adverse changes in lens transparency and biochemical markers were not prevented by an antioxidant (2 mM dithiothreitol) or nontoxic transport substrates (200 microM probenecid or 10 mm phenylalanine, S-benzyl-L-cysteine or para-aminohippuric acid). Calpain inhibitors E64d (100 microM) and calpain inhibitor II (1 mM) were ineffective in preventing opacity formation caused by DCVC. In a small separate study, DCVC toxicity to explanted lenses from cynomologus monkeys was also ameliorated by coincubation with AOA. These results indicate that opacity formation by DCVC in rodent and primate lenses in vitro is primarily mediated via lenticular beta-lyase metabolism of DCVC to a reactive metabolite. Metabolism of DCVC by FMO and perturbations in mitochondrial calcium (Ca2+) homeostasis and increased poly(ADP-ribosylation) of nuclear proteins may play a limited role in opacity formation in vitro. However, opacity formation does not appear to be the result of oxidative stress or calpain activation. DCVC toxicity to the lens was not blocked with competitive inhibitors of the amino acid and organic anion transporters of DCVC as is found in the kidney.

Precipitation of crystallins from young rat lens by endogenous calpain

The purpose of these experiments was to study the mechanism for precipitation of lens crystallins in cataract. An in vitro model was developed to activate the endogenous protease calpain II in the soluble proteins from young rat lens by addition of calcium in the presence of 120 mM KCl. Light-scattering, insoluble proteins were produced approximately 4-6 days after calpain II activation. Results showed that proteolysis was caused by activation of lens calpain II, proteolysis preceded precipitation by several days, and alpha-crystallin acted as a molecular chaperone against precipitation of crystallins caused by proteolysis. These data supported our hypothesis that calpain-induced proteolysis of the N-terminal arms of beta-crystallin polypeptides leads to a loss of normal oligomerization of beta-crystallin polypeptides and formation of abnormal insoluble aggregates, possibly stabilized by hydrophobic interactions.

Proteolysis by calpain is an underlying mechanism for formation of sugar cataract in rat lens

To confirm the effect of a new aldose reductase inhibitor (ARI), rat lenses were cultured with xylose. ARI prevented opacities and reduced lens hydration caused by xylose. Next, cataract was produced by feeding a diet containing 50% galactose. ARI was tested for amelioration of cataract. On day 19 after feeding of galactose, nuclear cataracts were visible in 75% of the animals receiving only galactose, while nuclear cataracts were not observed in animals treated with ARI. In galactose cataract, lens hydration and calcium were significantly increased. Calpain in soluble and insoluble fractions was decreased. Alpha- and beta-crystallins were proteolyzed. These changes were inhibited by administration of ARI. These results suggested that proteolysis by calpain is an underlying mechanism in formation of sugar cataract in rat lens.

Expression of the cysteine proteinase inhibitor cystatin C mRNA in rat eye

BACKGROUND

Cystatin C, a naturally occurring inhibitor of cysteine proteinases, belongs to family 2 of the cystatin superfamily. While cystatins in general, and cystatin C specifically, are expressed in various cell types and found in biological fluids, cystatins in ocular structures have not been investigated. In the present study, the expression of cystatin C mRNA in the eye of the rat was studied.

METHODS

Total RNA was extracted from eyes as well as from pooled corneae, retinas, lenses, sclerae, and corneae of adult rats. Cystatin C mRNA was detected in the RNA samples by reverse transcriptase--polymerase chain reaction and Northern blot hybridization. In addition, in situ hybridizations of formalin-fixed cryostat sections were carried out using a digoxigenin-labeled cystatin C probe.

RESULTS

Cystatin C mRNA was demonstrated in total RNAs extracted from the eye, sclera, and retina, but not in RNAs isolated from the cornea and lens. In situ hybridizations revealed cystatin C mRNA in most of the stromal cells of the sclera. In the retina, a strong signal was localized in the outer nuclear layer. The distribution of the reaction product suggested that in the retina Müller cells and rod cells are the primary sites of expression of cystatin C. In addition, some glial cells in the inner nuclear and ganglion cell layers were stained. No specific signal for cystatin C mRNA was detected in the cornea, lens, iris, ciliary body, and choroid.

CONCLUSIONS

In the eye of the rat, significant levels of cystatin C mRNA are detected in the sclera and retina. In the sclera cystatin C may play a role in modulating the activities of cysteine proteinases, mostly cathepsins, involved in the turnover and remodeling of the stroma. In the retina, cystatins synthesized and presumably released by Müller cells and rod cells may have a protective function against the harmful effects of cysteine proteinases released under physiologic and pathologic conditions.