Oxysterol Compounds in Mouse Mutant αA- and αB-Crystallin Lenses Can Improve the Optical Properties of the Lens

Purpose

To investigate how cataract-linked mutations affect the gradient refractive index (GRIN) and lens opacification in mouse lenses and whether there is any effect on the optics of the lens from treatment with an oxysterol compound.

Methods

A total of 35 mice including wild-type and knock-in mutants (Cryaa-R49C and Cryab-R120G) were used in these experiments: 26 mice were treated with topical VP1-001, an oxysterol, in one eye and vehicle in the other, and nine mice were untreated controls. Slit lamp biomicroscopy was used to analyze the lens in live animals and to provide apparent cataract grades. Refractive index in the lenses of 64 unfixed whole mouse eyes was calculated from measurements with X-ray phase tomography based on X-ray Talbot interferometry with a synchrotron radiation source.

Results

Heterozygous Cryaa-R49C lenses had slightly irregularly shaped contours in the center of the GRIN and distinct disturbances of the gradient index at the anterior and posterior poles. Contours near the lens surface were denser in homozygous Cryab-R120G lenses. Treatment with topical VP1-001, an oxysterol, showed an improvement in refractive index profiles in 61% of lenses and this was supported by a reduction in apparent lens opacity grade by 1.0 in 46% of live mice.

Conclusions

These results indicate that α-crystallin mutations alter the refractive index gradient of mouse lenses in distinct ways and suggest that topical treatment with VP1-001 may improve lens transparency and refractive index contours in some lenses with mutations.

Analysis of amyloid-like secondary structure in the Cryab-R120G knock-in mouse model of hereditary cataracts by two-dimensional infrared spectroscopy

αB-crystallin is a small heat shock protein that forms a heterooligomeric complex with αA-crystallin in the ocular lens. It is also widely distributed in tissues throughout the body and has been linked with neurodegenerative diseases such as Alzheimer's, where it is associated with amyloid fibrils. Crystallins can form amorphous aggregates in cataracts as well as more structured amyloid-like fibrils. The arginine 120 to glycine (R120G) mutation in αB-crystallin (Cryab-R120G) results in high molecular weight crystallin protein aggregates and loss of the chaperone activity of the protein in vitro, and it is associated with human hereditary cataracts and myopathy. Characterizing the amorphous (unstructured) versus the highly ordered (amyloid fibril) nature of crystallin aggregates is important in understanding their role in disease and important to developing pharmacological treatments for cataracts. We investigated protein secondary structure in wild-type (WT) and Cryab-R120G knock-in mutant mouse lenses using two-dimensional infrared (2DIR) spectroscopy, which has been used to detect amyloid-like fibrils in human lenses and measure UV radiation-induced changes in porcine lenses. Our goal was to compare the aggregated proteins in this mouse lens model to human lenses and evaluate the protein structural relevance of the Cryab-R120G knock-in mouse model to general age-related cataract disease. In the 2DIR spectra, amide I diagonal peak frequencies were red-shifted to smaller wavenumbers in mutant mouse lenses as compared to WT mouse lenses, consistent with an increase in ordered secondary structure. The cross peak frequency and intensity indicated the presence of amyloid in the mutant mouse lenses. While the diagonal and cross peak changes in location and intensity from the 2DIR spectra indicated significant structural differences between the wild type and mutant mouse lenses, these differences were smaller than those found in human lenses; thus, the Cryab-R120G knock-in mouse lenses contain less amyloid-like secondary structure than human lenses. The results of the 2DIR spectroscopy study confirm the presence of amyloid-like secondary structure in Cryab-R120G knock-in mice with cataracts and support the use of this model to study age-related cataract.

Alpha-crystallin mutations alter lens metabolites in mouse models of human cataracts

Cataracts are a major cause of blindness worldwide and commonly occur in individuals over 70 years old. Cataracts can also appear earlier in life due to genetic mutations. The lens proteins, αA- and αB-crystallins, are chaperone proteins that have important roles maintaining protein solubility to prevent cataract formation. Mutations in the CRYAA and CRYAB crystallin genes are associated with autosomal dominant early onset human cataracts. Although studies about the proteomic and genomic changes that occur in cataracts have been reported, metabolomics studies are very limited. Here, we directly investigated cataract metabolism using gas-chromatography-mass spectrometry (GC-MS) to analyze the metabolites in adult Cryaa-R49C and Cryab-R120G knock-in mouse lenses. The most abundant metabolites were myo-inositol, L-(+)-lactic acid, cholesterol, phosphate, glycerol phosphate, palmitic and 9-octadecenoic acids, α-D-mannopyranose, and β-D-glucopyranose. Cryaa-R49C knock-in mouse lenses had a significant decrease in the number of sugars and minor sterols, which occurred in concert with an increase in lactic acid. Cholesterol composition was unchanged. In contrast, Cryab-R120G knock-in lenses exhibited increased total amino acid content including valine, alanine, serine, leucine, isoleucine, glycine, and aspartic acid. Minor sterols, including cholest-7-en-3-ol and glycerol phosphate were decreased. These studies indicate that lenses from Cryaa-R49C and Cryab-R120G knock-in mice, which are models for human cataracts, have unique amino acid and metabolite profiles.

Changes in relative histone abundance and heterochromatin in αA-crystallin and αB-crystallin knock-in mutant mouse lenses

Objective

Understanding the mechanisms of cataract formation is important for age-related and hereditary cataracts caused by mutations in lens protein genes. Lens proteins of the crystallin gene families α-, β-, and γ-crystallin are the most abundant proteins in the lens. Single point mutations in crystallin genes cause autosomal dominant cataracts in multigenerational families. Our previous proteomic and RNAseq studies identified genes and proteins altered in the early stages of cataract formation in mouse models. Histones H2A, H2B, and H4 increase in abundance in αA- and αB-crystallin mutant mouse lenses and in cultured cells expressing the mutant form of αA-crystallin linked with hereditary cataracts.

Results

In this study of histones in mutant lenses, we extracted histones from adult mouse lenses from cryaa-R49C and cryab-R120G mutant knock-in mice. We characterized the histones using matrix-assisted laser desorption/ionization time of flight (MALDI-TOF)-mass spectrometric analysis and gel electrophoresis and characterized the lens nucleus morphology using electron microscopy (EM). The relative abundance of histone H3 protein decreased in lenses from cryaa-R49C mutant mice and the relative abundance of histone H2 increased in these lenses. Electron microscopy of nuclei from cryaa-R49C-homozygous mutant mouse lenses revealed a pronounced alteration in the distribution of heterochromatin.

Creatine kinase/α-crystallin interaction functions in cataract development

Creatine kinase (CK) is an energy storage enzyme that plays an important role in energy metabolism. CK/phosphocreatine functions as an energy buffer and links ATP production sites with ATP utilization sites. Several key mutations in the αA-crystallin (cryaa) and αB-crystallin (cryab) genes have been linked with autosomal-dominant, hereditary human cataracts. The cryaa-R49C mutation was identified in a four-generation Caucasian family. We previously identified an increase in the quantity of CK complexed with α-crystallin in the lenses of knock-in mice expressing the cryaa-R49C mutation using proteomic analyses. Increased levels of CK in postnatal cataractous lenses may indicate increased ATP requirements during early cataract development. To gain a further understanding of the relationship between CK and α-crystallin, we investigated whether α-crystallin interacts with and forms complexes with CK, in vitro. Isothermal titration calorimetry (ITC) showed that each CK dimer bound to 28 α-crystallin subunits, with a K_d of 3.3 × 10⁻⁷ M, and that the interaction between α-crystallin and CK was endothermic, thermodynamically favorable, and entropy-driven. High-salt concentrations did not affect the interaction between CK and α-crystallin, suggesting that the interaction between CK and α-crystallin is primarily hydrophobic. Gel permeation chromatography (GPC) detected water-soluble α-crystallin and CK complexes, as determined by increased light scattering after complex formation. In addition, CK and α-crystallin formed partially-water-insoluble, high-molecular-mass complexes. Enzyme-linked immunosorbent assay (ELISA)-based enzymatic activity analyses of lens homogenates showed a 17-fold increase in CK activity in the postnatal lenses of cryaa-R49C knock-in mice. These studies indicate that the interaction between α-crystallin and CK is functionally important and that increased CK levels may be necessary to meet the increased ATP demands of ATP-dependent functions in cataractous lenses.

•

Cataract model α-crystallin mutant mice exhibit upregulated creatine kinase.

•

Isothermal titration calorimetry detected creatine kinase/α-crystallin interaction.

•

The protein-protein interaction is thermodynamically favorable and entropy driven.

Mechanism of Action of VP1-001 in cryAB(R120G)-Associated and Age-Related Cataracts

Purpose

We previously identified an oxysterol, VP1-001 (also known as compound 29), that partially restores the transparency of lenses with cataracts. To understand the mechanism of VP1-001, we tested the ability of its enantiomer, ent-VP1-001, to bind and stabilize αB-crystallin (cryAB) in vitro and to produce a similar therapeutic effect in cryAB(R120G) mutant and aged wild-type mice with cataracts. VP1-001 and ent-VP1-001 have identical physicochemical properties. These experiments are designed to critically evaluate whether stereoselective binding to cryAB is required for activity.

Methods

We compared the binding of VP1-001 and ent-VP1-001 to cryAB using in silico docking, differential scanning fluorimetry (DSF), and microscale thermophoresis (MST). Compounds were delivered by six topical administrations to mouse eyes over 2 weeks, and the effects on cataracts and lens refractive measures in vivo were examined. Additionally, lens epithelial and fiber cell morphologies were assessed via transmission electron microscopy.

Results

Docking studies suggested greater binding of VP1-001 into a deep groove in the cryAB dimer compared with ent-VP1-001. Consistent with this prediction, DSF and MST experiments showed that VP1-001 bound cryAB, whereas ent-VP1-001 did not. Accordingly, topical treatment of lenses with ent-VP1-001 had no effect, whereas VP1-001 produced a statistically significant improvement in lens clarity and favorable changes in lens morphology.

Conclusions

The ability of VP1-001 to bind native cryAB dimers is important for its ability to reverse lens opacity in mouse models of cataracts.

In vitro interactions of histones and α-crystallin

The aggregation of crystallins in lenses is associated with cataract formation. We previously reported that mutant crystallins are associated with an increased abundance of histones in knock-in and knockout mouse models. However, very little is known about the specific interactions between lens crystallins and histones. Here, we performed in vitro analyses to determine whether α-crystallin interacts with histones directly. Isothermal titration calorimetry revealed a strong histone–α-crystallin binding with a K_d of 4 × 10⁻⁷ M, and the thermodynamic parameters suggested that the interaction was both entropy and enthalpy driven. Size-exclusion chromatography further showed that histone–α-crystallin complexes are water soluble but become water insoluble as the concentration of histones is increased. Right-angle light scattering measurements of the water-soluble fractions of histone–α-crystallin mixtures showed a decrease in the oligomeric molecular weight of α-crystallin, indicating that histones alter the oligomerization of α-crystallin. Taken together, these findings reveal for the first time that histones interact with and affect the solubility and aggregation of α-crystallin, indicating that the interaction between α-crystallin and histones in the lens is functionally important.

•

Histones are upregulated in α-crystallin mutant mice with hereditary cataracts.

•

Isothermal titration calorimetry reveals histones and α-crystallin interact.

•

These interactions are enthalpy and entropy driven.

•

Histones affect the solubility and aggregation behavior of α-crystallin in vitro.

Probing the changes in gene expression due to α-crystallin mutations in mouse models of hereditary human cataract

The mammalian eye lens expresses a high concentration of crystallins (α, β and γ-crystallins) to maintain the refractive index essential for lens transparency. Crystallins are long-lived proteins that do not turnover throughout life. The structural destabilization of crystallins by UV exposure, glycation, oxidative stress and mutations in crystallin genes leads to protein aggregation and development of cataracts. Several destabilizing mutations in crystallin genes are linked with human autosomal dominant hereditary cataracts. To investigate the mechanism by which the α-crystallin mutations Cryaa-R49C and Cryab-R120G lead to cataract formation, we determined whether these mutations cause an altered expression of specific transcripts in the lens at an early postnatal age by RNA-seq analysis. Using knock-in mouse models previously generated in our laboratory, in the present work, we identified genes that exhibited altered abundance in the mutant lenses, including decreased transcripts for Clic5, an intracellular water channel in Cryaa-R49C heterozygous mutant lenses, and increased transcripts for Eno1b in Cryab-R120G heterozygous mutant lenses. In addition, RNA-seq analysis revealed increased histones H2B, H2A, and H4 gene expression in Cryaa-R49C mutant lenses, suggesting that the αA-crystallin mutation regulates histone expression via a transcriptional mechanism. Additionally, these studies confirmed the increased expression of histones H2B, H2A, and H4 by proteomic analysis of Cryaa-R49C knock-in and Cryaa;Cryab gene knockout lenses reported previously. Taken together, these findings offer additional insight into the early transcriptional changes caused by Cryaa and Cryab mutations associated with autosomal dominant human cataracts, and indicate that the transcript levels of certain genes are affected by the expression of mutant α-crystallin in vivo.

Pharmacological chaperone for α-crystallin partially restores transparency in cataract models

Cataracts reduce vision in 50% of individuals over 70 years of age and are a common form of blindness worldwide. Cataracts are caused when damage to the major lens crystallin proteins causes their misfolding and aggregation into insoluble amyloids. Using a thermal stability assay, we identified a class of molecules that bind α-crystallins (cryAA and cryAB) and reversed their aggregation in vitro. The most promising compound improved lens transparency in the R49C cryAA and R120G cryAB mouse models of hereditary cataract. It also partially restored protein solubility in the lenses of aged mice in vivo and in human lenses ex vivo. These findings suggest an approach to treating cataracts by stabilizing α-crystallins.

Autophagy and UPR in alpha-crystallin mutant knock-in mouse models of hereditary cataracts

BACKGROUND

Knock-in mice provide useful models of congenital and age-related cataracts caused by α-crystallin mutations. R49C αA-crystallin and R120G αB-crystallin mutations are linked with hereditary cataracts. Knock-in αA-R49C+/- heterozygotes develop cataracts by 1-2months, whereas homozygote mice have cataracts at birth. The R49C mutation drastically reduces lens protein water solubility and causes cell death in knock-in mouse lenses. Mutant crystallin cannot function as a chaperone, which leads to protein aggregation and lens opacity. Protein aggregation disrupts the lens fiber cell structure and normal development and causes cell death in epithelial and fiber cells. We determined what aspects of the wild-type phenotype are age-dependently altered in the mutant lens.

METHODS

Wild-type, heterozygote (αA-R49C+/-), and homozygote (αA-R49C+/+) mouse lenses were assessed pre- and postnatally for lens morphology (electron microscopy, immunohistochemistry), and autophagy or unfolded protein response markers (immunoblotting).

RESULTS

Morphology was altered by embryonic day 17 in R49C+/+ lenses; R49C+/- lens morphology was unaffected at this stage. Active autophagy in the lens epithelium of mutant lenses was indicated by the presence of autophagosomes using electron microscopy. Protein p62 levels, which are degraded specifically by autophagy, increased in αA-R49C mutant versus wild-type lenses, suggesting autophagy inhibition in the mutant lenses. The unfolded protein response marker XBP-1 was upregulated in adult lenses of αB-R120G+/+ mice, suggesting its role in lens opacification.

CONCLUSIONS

Mutated crystallins alter lens morphology, autophagy, and stress responses.

GENERAL SIGNIFICANCE

Therapeutic modulation of autophagic pathways may improve protein degradation in cataractous lenses and reduce lens opacity. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

In vivo substrates of the lens molecular chaperones αA-crystallin and αB-crystallin

αA-crystallin and αB-crystallin are members of the small heat shock protein family and function as molecular chaperones and major lens structural proteins. Although numerous studies have examined their chaperone-like activities in vitro, little is known about the proteins they protect in vivo. To elucidate the relationships between chaperone function, substrate binding, and human cataract formation, we used proteomic and mass spectrometric methods to analyze the effect of mutations associated with hereditary human cataract formation on protein abundance in αA-R49C and αB-R120G knock-in mutant lenses. Compared with age-matched wild type lenses, 2-day-old αA-R49C heterozygous lenses demonstrated the following: increased crosslinking (15-fold) and degradation (2.6-fold) of αA-crystallin; increased association between αA-crystallin and filensin, actin, or creatine kinase B; increased acidification of βB1-crystallin; increased levels of grifin; and an association between βA3/A1-crystallin and αA-crystallin. Homozygous αA-R49C mutant lenses exhibited increased associations between αA-crystallin and βB3-, βA4-, βA2-crystallins, and grifin, whereas levels of βB1-crystallin, gelsolin, and calpain 3 decreased. The amount of degraded glutamate dehydrogenase, α-enolase, and cytochrome c increased more than 50-fold in homozygous αA-R49C mutant lenses. In αB-R120G mouse lenses, our analyses identified decreased abundance of phosphoglycerate mutase, several β- and γ-crystallins, and degradation of αA- and αB-crystallin early in cataract development. Changes in the abundance of hemoglobin and histones with the loss of normal α-crystallin chaperone function suggest that these proteins also play important roles in the biochemical mechanisms of hereditary cataracts. Together, these studies offer a novel insight into the putative in vivo substrates of αA- and αB-crystallin.

p62 expression and autophagy in αB-crystallin R120G mutant knock-in mouse model of hereditary cataract

The formation of cataracts is associated with the accumulation of protein aggregates in the ocular lens, suggesting that defective protein degradation plays a role in cataract pathogenesis. Accumulation of the p62 protein has recently been identified as a marker for impaired autophagy in a variety of tissues; however, little information exists on its expression in the ocular lens and in cataracts. In the present study we examined the expression of p62 in the mouse lens and compared its expression in wild-type lenses with that in lenses from knock-in mice with an arginine to glycine mutation in αB-crystallin (αB-R120G) that is known to cause human hereditary cataract. Immunohistochemical, immunoblotting, and transmission electron microscopic analyses of wild type and αB-R120G mutant mice were performed. To assess the effect of increased protein aggregation on autophagy, immunohistochemical staining was performed with an anti-p62 antibody, revealing the presence of p62-positive punctate staining in a band of denucleated cortical fiber cells. The number and size of p62 puncta were significantly greater in αB-R120G homozygous mutant lenses than in wild type and heterozygous mutant lenses. p62 staining was also abundant in lens epithelial cells and was concentrated around the nuclear membrane. Double-membraned structures similar to autophagosomes containing cellular cytoplasmic content were detected in lens epithelial cells by transmission electron microscopy. The autophagosomes in lens epithelial cells from αB-R120G homozygous mutant mice were larger than those in wild type mice. Double-membraned structures that are probably autophagosomes were also detected in cortical fiber cells and were more abundant in the αB-R120G homozygous mutant lens than the wild type lens. This study demonstrates p62 distribution as speckles in the lens fiber cells, altered levels of p62 expression, and the presence of autophagosomes in the ocular lens of αB-R120G mutant mice. We propose that autophagy is inhibited in the αB-R120G mutant lenses because of a defect in protein degradation after autophagosome formation. Further work is necessary to determine the relationship between αB-crystallin function, autophagy, and cataract formation.

Comparative proteomic analysis identifies age-dependent increases in the abundance of specific proteins after deletion of the small heat shock proteins αA- and αB-crystallin

Mice with deletion of genes for small heat shock proteins αA- and αB-crystallin (αA/αB(-/-)) develop cataracts. We used proteomic analysis to identify lens proteins that change in abundance after deletion of these α-crystallin genes. Wild-type (WT) and αA/αB(-/-) knockout (DKO) mice were compared using two-dimensional difference gel electrophoresis and mass spectrometric analysis, and protein identifications were validated by Mascot proteomic software. The abundance of histones H2A, H4, and H2B fragment, and a low molecular weight β1-catenin increased 2-3-fold in postnatal day 2 lenses of DKO lenses compared with WT lenses. Additional major increases were observed in abundance of βB2-crystallin and vimentin in 30-day-old lenses of DKO animals compared with WT animals. Lenses of DKO mice were comprised of nine protein spots containing βB2-crystallin at 10-40-fold higher abundance and three protein spots containing vimentin at ≥2-fold higher abundance than in WT lenses. Gel permeation chromatography identified a unique 328 kDa protein in DKO lenses, containing β-crystallin, demonstrating aggregation of β-crystallin in the absence of α-crystallins. Together, these changes provide biochemical evidence for possible functions of specific cell adhesion proteins, cytoskeletal proteins, and crystallins in lens opacities caused by the absence of the major chaperones, αA- and αB-crystallins.

Inhibition of lens photodamage by UV-absorbing contact lenses

PURPOSE

To determine whether class 1 UV-blocking contact lenses protect against UVB radiation-induced damage in a human lens epithelial cell line (HLE B-3) and postmortem human lenses using a proteomics approach.

METHODS

HLE B-3 cells were exposed to 6.4 mW/cm(2) UVB radiation at 302 nm for 2 minutes (768 mJ/cm(2)) with or without covering by senofilcon A class 1 UV-blocking contact lenses or lotrafilcon A non-UV-blocking (lotrafilcon A has some UV-blocking ability, albeit minimal) contact lenses. Control cells were not exposed to UVB radiation. Four hours after treatment, cells were analyzed by two-dimensional difference gel electrophoresis and tandem mass spectrometry, and changes in protein abundance were quantified. F-actin and microtubule cytoskeletons were examined by fluorescence staining. In addition, human donor lenses were exposed to UVB radiation at 302 nm for 4 minutes (1536 mJ/cm(2)). Cortical and epithelial cell proteins were scraped from lens surfaces and subjected to the same protein analyses.

RESULTS

Senofilcon A lenses were beneficial for protecting HLE B-3 cells against UVB radiation-induced changes in caldesmon 1 isoform, lamin A/C transcript variant 1, DEAD (Asp-Glu-Ala-Asp) box polypeptide, β-actin, glyceraldehyde 3-phosphate dehydrogenase (G3PDH), annexin A2, triose phosphate isomerase, and ubiquitin B precursor. These contact lenses also prevented actin and microtubule cytoskeleton changes typically induced by UVB radiation. Conversely, non-UV-blocking contact lenses were not protective. UVB-irradiated human lenses showed marked reductions in αA-crystallin, αB-crystallin, aldehyde dehydrogenase 1, βS-crystallin, βB2-crystallin, and G3PDH, and UV-absorbing contact lenses significantly prevented these alterations.

CONCLUSIONS

Senofilcon A class 1 UV-blocking contact lenses largely prevented UVB-induced changes in protein abundance in lens epithelial cells and in human lenses.

Immunological detection of N-formylkynurenine in porphyrin-mediated photooxided lens α-crystallin

Crystallin proteins are responsible for maintaining lens transparency and allowing the lens to focus light undistorted onto the retina. The α-crystallins are the major lens crystallins, and function as both structural proteins and chaperones to protect all lens proteins from damage leading to lens deterioration. Because lens crystallin proteins do not turn over, the damage they accumulate can lead to cataracts, the world's leading cause of blindness. Photosensitizing porphyrins can accumulate in the eye through either endogenous metabolism or through therapeutic or diagnostic procedures. Porphyrin buildup exacerbates lens aging through increased levels of singlet oxygen, resulting in protein polymerization and amino acid residue alteration. Tryptophans oxidize to kynurenine and N-formylkynurenine (NFK) causing irreversible changes in the refractive index of the normally transparent lens, leading to development of cataracts. Additionally, NFK is itself a photosensitizer, and its presence exacerbates lens deterioration. This work uses anti-NFK antiserum to study porphyrin-facilitated photooxidation of α-crystallin tryptophan residues. In vitro experiments show that four biologically interesting porphyrins mediate α-crystallin polymerization and accumulation of both protein radicals and NFK. Confocal microscopy of cultured human lens epithelial cells indicates that while all four porphyrins photosensitize cellular proteins, not all oxidize the tryptophans of cellular α-crystallin to NFK.

Evaluation of the toxicity of triamcinolone acetonide and dexamethasone sodium phosphate on human lens epithelial cells (HLE B-3)

PURPOSE

The purpose of this study was to compare the in vitro effects of triamcinolone acetonide (TA) and dexamethasone sodium phosphate (DEX) on human lens epithelial cells (HLE B-3).

METHODS

HLE B-3 cells were exposed for 24 h to commercially available TA (c-TA) and dimethylsulfoxide-solubilized TA (s-TA). The cells were treated with 1,000 (clinical dose), 750, 500, 200, and 100 μg/mL concentrations of c-TA, s-TA, and supernatant for 24 h. The cells were also treated with DEX at 2, 1, 0.5, 0.2, 0.1 (clinical dose), and 0.05 mg/mL. Cell viability, caspase-3/7 activity, and DNA fragmentation analyses were performed.

RESULTS

The mean cell viabilities of HLE B-3 after exposure to c-TA at 1,000, 750, 500, 200, and 100 μg/mL were significantly reduced compared with control untreated cells. The s-TA also significantly reduced cell viability at 1,000, 750, and 500 μg/mL compared with dimethylsulfoxide control. The supernatant did not reduce cell viability. Caspase-3/7 activity significantly increased after treatment with c-TA and s-TA. DNA laddering revealed bands at 200 bp intervals with both c-TA at≥100 μg/mL and s-TA at ≥500 μg/mL. The cell viabilities of HLE B-3 after 24 h exposure to DEX were significantly reduced at 2 and 1 mg/mL but not at lower concentrations tested. Caspase-3/7 activities in HLE B-3 cells were not increased significantly after treatment with DEX at any dose tested. DNA laddering did not reveal any band at any dose tested.

CONCLUSION

This study showed that TA at its clinical dose (1,000 μg/mL) in both commercial preparation and solubilized forms decrease HLE B-3 cell viability through an apoptotic pathway. DEX at its clinical dose (0.1 mg/mL) does not decrease cell viability or cause any increase of caspase-3/7 activity. This study suggests that for long-term sustained-release devices, DEX may be less damaging to human lens cells than TA.

Activation of the unfolded protein response by a cataract-associated αA-crystallin mutation

αA-crystallin is a lens chaperone that plays an essential role in the transparency and refractive properties of the lens. Mutations in αA-crystallin have been associated with the development of hereditary cataracts. The R49C mutation of αA-crystallin (αA-R49C) was identified in a four-generation Caucasian family with hereditary cataracts. The αA-R49C protein forms larger-than-normal oligomers in the lens and has decreased solubility. This aberrant αA-R49C oligomerization suggests that protein folding is altered. However, whether activation of the unfolded protein response (UPR) occurs during crystallin mutation-induced cataract formation and whether the UPR causes cell death under these conditions is unclear. We investigated UPR activation in an in vivo mouse model of αA-R49C using immunoblot analysis of lens extracts. We found that expression of the endoplasmic reticulum (ER) chaperone, BiP, was 5-fold higher in homozygous αA-R49C lenses than in wild type lenses. Analysis of proteins typically expressed during the UPR revealed that ATF-4 and CHOP levels were also higher in homozygous lenses than in wild type lenses, while the opposite was true of ATF-6 and XBP-1. Taken together, these findings show that mutation of αA-crystallin induces activation of the UPR during cataract formation. They also suggest that the UPR is an important mediator of cell death observed in homozygous αA-R49C lenses.

αA-Crystallin associates with α6 integrin receptor complexes and regulates cellular signaling

α-Crystallins are small heat-shock proteins important to lens transparency that provide the lens with its refractive properties. In their role as molecular chaperones, these crystallins also prevent protein aggregation, affect cytoskeletal remodeling, enhance resistance to cell stress, and provide lens cells with protection against apoptosis. While many of the functions assigned to αA-crystallin are attributable to its presence in the cytoplasm of lens cells, αA-crystallin also has been detected at the lens plasma membrane. However, how αA-crystallin becomes linked to the plasma membrane or what its functions are at this site has remained unknown. In this study, we examined the mechanisms by which αA-crystallin becomes associated with the lens membrane, focusing specifically on its interaction with membrane receptors, and the differentiation-specificity of these interactions. We also determined how the long-term absence of αA-crystallin alters receptor-linked signaling pathways. αA-crystallin association with membrane receptors was determined by co-immunoprecipitation analysis; its membrane localization was examined by confocal imaging; and the effect of αA-crystallin loss-of-function on the activation state of signaling molecules in pathways linked to membrane receptors was determined by immunoblot analysis. The results show that, in lens epithelial cells, plasma membrane αA-crystallin was primarily localized to apicolateral borders, reflecting the association of αA-crystallin with E-cadherin complexes. These studies also provide the first evidence that αA-crystallin maintained its association with the plasma membrane in lens cortical fiber cells, where it was localized to lateral interfaces, and further show that this association was mediated, in part, by αA-crystallin interaction with α6 integrin receptor complexes. We report that the absence of αA-crystallin led to constitutive activation of the stress kinases p38 and JNK, classical inducers of apoptotic cell death, and the loss of the phospho-Bad pro-survival signal, effects that were greatest in differentiating lens fiber cells. Concurrent with this, activation of FAK and ERK kinases was increased, demonstrating that these receptor-linked pathways also were dysregulated in the absence of αA-crystallin. These data link αA-crystallin plasma membrane association to its differentiation-state-specific interaction with E-cadherin and α6 integrin receptor complexes. The changes in cell signaling in αA-crystallin-null lenses suggest that dysregulation of receptor-linked cell-signaling pathways that accompany the failure of αA-crystallin to associate with membrane receptors may be responsible for the induction of apoptosis. The observed changes in lens cell signaling likely reflect long-term functional adaptations to the absence of the αA-crystallin chaperone/small heat-shock protein.

Quantitative biometric phenotype analysis in mouse lenses

The disrupted morphology of lenses in mouse models for cataracts precludes accurate in vitro assessment of lens growth by weight. To overcome this limitation, we developed morphometric methods to assess defects in eye lens growth and shape in mice expressing the alphaA-crystallin R49C (alphaA-R49C) mutation. Our morphometric methods determine quantitative shape and dry weight of the whole lens from histological sections of the lens. This method was then used to quantitatively compare the biometric growth patterns of lenses of different genotypes of mice from birth to 12 months. The wild type dry lens weights determined using the morphometric method were comparable to previously reported weights. Next we applied the method to assessing the lenses of alphaA-R49C knock-in mice, which exhibit decreased alphaA-crystallin protein solubility, resulting in a variety of growth abnormalities including early cataract formation, decreased eye and lens size, failure to form the equatorial bow region, and continued lens cell death, sometimes resulting in the entire loss of the lens and eye. Our morphometric methods reproducibly quantified these defects by combining histology, microscopy, and image analysis. The volume measurement accurately represented the total growth of the lens, whereas the geometric shape of the lens more accurately quantified the differences between the growth of the mutant and wild-type lenses. These methods are robust tools for measuring dry lens weight and quantitatively comparing the growth of small lenses that are difficult to weigh accurately such as those from very young mice and mice with developmental lens defects.

Detection and prevention of ocular phototoxicity of ciprofloxacin and other fluoroquinolone antibiotics

Fluoroquinolone (FLQ) drugs are a potent family of antibiotics used to treat infections including ocular infections. To determine if these antibiotics may be phototoxic to the eye, we exposed human lens epithelial cells to 0.125-1 mm FLQs (ciprofloxacin [Cipro], lomefloxacin [Lome], norfloxacin [Nor] and ofloxacin [Ofl]), the precursor quinolone nalidixic acid (Nalid) and UVA radiation (2.5 J cm(-2)). Based on fluorescence confocal microscopy, FLQs are diffused throughout the cytoplasm and preferentially located in the lysosomes of lens epithelial cells. Neither FLQ exposure alone nor UVA exposure alone reduced cell viability. However, with exposure to UVA radiation the FLQs studied (Cipro, Nor, Lome and Ofl) induced a phototoxic reaction that included necrosis, apoptosis, loss of cell viability as measured by MTS, and membrane damage as determined by the lactate dehydrogenase assay. Both Nalid and all FLQs studied (Cipro, Nor, Lome and Ofl) photopolymerized the lens protein alpha-crystallin. Phototoxic damage to lens epithelial cells and/or alpha-crystallin will lead to a loss of transparency of the human lens. However, if precautions are taken to filter all UV radiation from the eye while taking these antibiotics, eye damage may be prevented.

In vivo lens deficiency of the R49C alphaA-crystallin mutant

The R49C mutation of alphaA-crystallin (alphaA-R49C) causes hereditary cataracts in humans; patients in a four-generation Caucasian family were found be heterozygous for this autosomal dominant mutation. We previously generated knock-in mouse models of this mutation and found that by 2 months of age, heterozygous mutant mice exhibited minor lens defects including reduced protein solubility, altered signaling in epithelial and fiber cells, and aberrant interactions between alphaA-crystallin and other lens proteins. In contrast, homozygous mutant alphaA-R49C knock-in mice displayed earlier and more extensive lens defects including small eyes and small lenses at birth, death of epithelial and fiber cells, and the formation of posterior, nuclear, and cortical cataracts in the first month of life. We have extended this study to now show that in alphaA-R49C homozygous mutant mice, epithelial cells failed to form normal equatorial bow regions and fiber cells continued to die as the mice aged, resulting in a complete loss of lenses and overall eye structure in mice older than 4 months. These results demonstrate that expression of the hereditary R49C mutant of alphaA-crystallin in vivo is sufficient to adversely affect lens growth, lens cell morphology, and eye function. The death of fiber cells caused by this mutation may ultimately lead to loss of retinal integrity and blindness.

Effects of alpha-crystallin on lens cell function and cataract pathology

The development of cataracts is a debilitating eye condition which is common in elderly patients and afflicts millions worldwide. Cataracts result from the deposition of aggregated proteins in the eye which causes clouding of the lens, light scattering, and obstruction of vision. Non-syndromic, hereditary human cataract development is linked to point mutations in the CRYAA and CRYAB genes which encode alphaA and alphaB-crystallin. The alpha-crystallins are small heat shock proteins which play central roles in maintaining lens transparency and refractive properties. The discovery in 1992 that these proteins possess chaperone-like activity has led most researchers to focus on the ability of alpha-crystallins to prevent protein aggregation in vitro. While the ability of alpha-crystallins to efficiently trap aggregation-prone denatured proteins in vitro is thought to delay the development of age-related cataracts in vivo, alpha-crystallins have additional functions which may also contribute to cataract pathology. In addition to chaperone activity, alpha-crystallins are known to protect cells from stress-induced apoptosis, regulate cell growth, and enhance genomic stability. They also physically and functionally interact with both the cell membrane and cytoskeleton. Functional changes in alpha-crystallin have been shown to modify membrane and cell-cell interactions and lead to lens cell pathology in vivo. This article focuses on the multiple diverse roles of alphaA-crystallin in the maintenance of lens function and cataract development in vivo.

AlphaA-crystallin R49Cneo mutation influences the architecture of lens fiber cell membranes and causes posterior and nuclear cataracts in mice

Background

αA-crystallin (CRYAA/HSPB4), a major component of all vertebrate eye lenses, is a small heat shock protein responsible for maintaining lens transparency. The R49C mutation in the αA-crystallin protein is linked with non-syndromic, hereditary human cataracts in a four-generation Caucasian family.

Methods

This study describes a mouse cataract model generated by insertion of a neomycin-resistant (neo^r) gene into an intron of the gene encoding mutant R49C αA-crystallin. Mice carrying the neo^r gene and wild-type Cryaa were also generated as controls. Heterozygous knock-in mice containing one wild type gene and one mutated gene for αA-crystallin (WT/R49C^neo) and homozygous knock-in mice containing two mutated genes (R49C^neo/R49C^neo) were compared.

Results

By 3 weeks, WT/R49C^neo mice exhibited large vacuoles in the cortical region 100 μm from the lens surface, and by 3 months posterior and nuclear cataracts had developed. WT/R49C^neo mice demonstrated severe posterior cataracts at 9 months of age, with considerable posterior nuclear migration evident in histological sections. R49C^neo/R49C^neo mice demonstrated nearly complete lens opacities by 5 months of age. In contrast, R49C mice in which the neo^r gene was deleted by breeding with CreEIIa mice developed lens abnormalities at birth, suggesting that the neo^r gene may suppress expression of mutant R49C αA-crystallin protein.

Conclusion

It is apparent that modification of membrane and cell-cell interactions occurs in the presence of the αA-crystallin mutation and rapidly leads to lens cell pathology in vivo.

Mechanism of insolubilization by a single-point mutation in alphaA-crystallin linked with hereditary human cataracts

AlphaA-crystallin is a small heat shock protein that functions as a molecular chaperone and a lens structural protein. The R49C single-point mutation in alphaA-crystallin causes hereditary human cataracts. We have previously investigated the in vivo properties of this mutant in a gene knock-in mouse model. Remarkably, homozygous mice carrying the alphaA-R49C mutant exhibit nearly complete lens opacity concurrent with small lenses and small eyes. Here we have investigated the 90 degrees light scattering, viscosity, refractive index, and bis-ANS fluorescence of lens proteins isolated from the alphaA-R49C mouse lenses and found that the concentration of total water-soluble proteins showed a pronounced decrease in alphaA-R49C homozygous lenses. Light scattering measurements on proteins separated by gel permeation chromatography showed a small amount of high-molecular mass aggregated material in the void volume which still remains soluble in alphaA-R49C homozygous lens homogenates. An increased level of binding of beta- and gamma-crystallin to the alpha-crystallin fraction was observed in alphaA-R49C heterozygous and homozygous lenses but not in wild-type lenses. Quantitative analysis with the hydrophobic fluorescence probe bis-ANS showed a pronounced increase in fluorescence yield upon binding to alpha-crystallin from mutant as compared with the wild-type lenses. These results suggest that the decrease in the solubility of the alphaA-R49C mutant protein was due to an increase in its hydrophobicity and supra-aggregation of alphaA-crystallin that leads to cataract formation. Our study further shows that analysis of mutant proteins from the mouse model is an effective way to understand the mechanism of protein insolubilization in hereditary cataracts.

Non-thermal electromagnetic radiation damage to lens epithelium

High frequency microwave electromagnetic radiation from mobile phones and other modern devices has the potential to damage eye tissues, but its effect on the lens epithelium is unknown at present. The objective of this study was to investigate the non-thermal effects of high frequency microwave electromagnetic radiation (1.1GHz, 2.22 mW) on the eye lens epithelium in situ. Bovine lenses were incubated in organ culture at 35°C for 10-15 days. A novel computer-controlled microwave source was used to investigate the effects of microwave radiation on the lenses. 58 lenses were used in this study. The lenses were divided into four groups: (1) Control lenses incubated in organ culture for 10 to15 days. (2) Electromagnetic radiation exposure group treated with 1.1 GHz, 2.22 mW microwave radiation for 90 cycles of 50 minutes irradiation followed by 10 minutes pause and cultured up to 10 days. (3) Electromagnetic radiation exposure group treated as group 2 with 192 cycles of radiation and cultured for 15 days. (4) Lenses exposed to 39.5ºC for 2 hours 3 times with 24 hours interval after each treatment beginning on the second day of the culture and cultured for 11 days. During the culture period, lens optical quality was followed daily by a computer-operated scanning laser beam. At the end of the culture period, control and treated lenses were analyzed morphologically and by assessment of the lens epithelial ATPase activity. Exposure to 1.1 GHz, 2.22 mW microwaves caused a reversible decrease in lens optical quality accompanied by irreversible morphological and biochemical damage to the lens epithelial cell layer. The effect of the electromagnetic radiation on the lens epithelium was remarkably different from those of conductive heat. The results of this investigation showed that electromagnetic fields from microwave radiation have a negative impact on the eye lens. The lens damage by electromagnetic fields was distinctly different from that caused by conductive heat.

Mechanism of small heat shock protein function in vivo: a knock-in mouse model demonstrates that the R49C mutation in alpha A-crystallin enhances protein insolubility and cell death

alphaA-crystallin (Cryaa/HSPB4) is a small heat shock protein and molecular chaperone that prevents nonspecific aggregation of denaturing proteins. Several point mutations in the alphaA-crystallin gene cause congenital human cataracts by unknown mechanisms. We took a novel approach to investigate the molecular mechanism of cataract formation in vivo by creating gene knock-in mice expressing the arginine 49 to cysteine mutation (R49C) in alphaA-crystallin (alphaA-R49C). This mutation has been linked with autosomal dominant hereditary cataracts in a four-generation Caucasian family. Homologous recombination in embryonic stem cells was performed using a plasmid containing the C to T transition in exon 1 of the cryaa gene. alphaA-R49C heterozygosity led to early cataracts characterized by nuclear opacities. Unexpectedly, alphaA-R49C homozygosity led to small eye phenotype and severe cataracts at birth. Wild type littermates did not show these abnormalities. Lens fiber cells of alphaA-R49C homozygous mice displayed an increase in cell death by apoptosis mediated by a 5-fold decrease in phosphorylated Bad, an anti-apoptotic protein, but an increase in Bcl-2 expression. However, proliferation measured by in vivo bromodeoxyuridine labeling did not decline. The alphaA-R49C heterozygous and homozygous knock-in lenses demonstrated an increase in insoluble alphaA-crystallin and alphaB-crystallin and a surprising increase in expression of cytoplasmic gamma-crystallin, whereas no changes in beta-crystallin were observed. Co-immunoprecipitation analysis showed increased interaction between alphaA-crystallin and lens substrate proteins in the heterozygous knock-in lenses. To our knowledge this is the first knock-in mouse model for a crystallin mutation causing hereditary human cataract and establishes that alphaA-R49C promotes protein insolubility and cell death in vivo.

The lens epithelium: focus on the expression and function of the alpha-crystallin chaperones

Lens epithelial cells are the parental cells responsible for growth and development of the transparent ocular lens. Many elegant investigations into their biology have focused on the factors that initiate and regulate lens epithelial cell differentiation. Because they serve key transport and cell maintenance functions throughout life, and are the primary source of metabolic activity in the lens, mechanisms to maintain lens epithelial cell integrity and survival are critical for lens transparency. The molecular chaperones alpha-crystallins are abundant proteins synthesized in the differentiated lens fiber cell cytoplasm. However, their expression in lens epithelial cells has only been appreciated very recently. Besides their important roles in the refractive and light focusing properties of the lens, alpha-crystallins have been implicated in a number of non-refractive pathways including those involving stress response, apoptosis and cell survival. The most convincing evidence for their importance in the lens epithelium has been shown by studies on the properties of lens epithelial cells from alphaA and alphaB-crystallin gene knockout mice. Novel combination of genetics, cell and molecular biology should lead to a greater understanding of how lens epithelial cells proliferate, differentiate and survive.

Up-regulation of tau, a brain microtubule-associated protein, in lens cortical fractions of aged alphaA-, alphaB-, and alphaA/B-crystallin knockout mice

PURPOSE

Alpha-crystallin is expressed at high levels in the lens in a complex of alphaA- and alphaB-crystallin subunits in 3:1 molar ratios, and is known to maintain the solubility of unpolymerized tubulin and enhance the resistance of microtubules to depolymerization, but its effect on proteins classically associated with microtubule stability (microtubule associated proteins) in the lens is unknown. In the present study we examined the expression of the brain microtubule associated protein tau in lenses of alpha-crystallin gene knockout mice.

METHODS

Quantitative RT-PCR, immunoblotting, cryo-immunoelectron microscopic and immunohistochemical methods were used to characterize the expression of tau in the lenses of alphaA(-/-)-, alphaB(-/-)-, and alphaA/B(-/-)-crystallin mice.

RESULTS

Immunoreactivity to tau, a 45-66 kDa brain microtubule associated protein that has been best characterized in neurons and neuronal pathologies, was uniquely upregulated in lens cortical fiber cells with aging and was associated with the microtubule fraction of alphaA(-/-)-, alphaB(-/-)-, and alphaA/B(-/-)-crystallin mouse lenses, but was undetectable in wild type lenses. Quantitative RT-PCR analysis further showed an upregulation of tau transcripts in alphaA(-/-)- and alphaA/B(-/-)-crystallin lenses. Brain microtubule fractions served as a positive control for tau in these experiments. An increase in phosphorylation of tau was detected in alphaA(-/-)- and alphaB(-/-)-crystallin brain proteins.

CONCLUSIONS

Although tau aggregation and alphaB-crystallin expression have been shown to increase in neurodegenerative diseases, surprisingly tau expression increases in the alpha-crystallin knockout lenses, suggesting that alphaA- and alphaB-crystallins are potentially important regulators of tau expression in lens.

Crystallins in the eye: Function and pathology

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. They were first discovered outside the vertebrate eye lens by Bhat and colleagues in 1989 who found alphaB-crystallin expression in the retina, heart, skeletal muscles, skin, brain and other tissues. With the advent of microarray and proteome analysis, there is a clearer demonstration that crystallins are prominent proteins both in the normal retina and in retinal pathologies, emphasizing the importance of understanding crystallin functions outside of the lens. There are two main crystallin gene families: alpha-crystallins, and betagamma-crystallins. alpha-crystallins are molecular chaperones that prevent aberrant protein interactions. The chaperone properties of alpha-crystallin are thought to allow the lens to tolerate aging-induced deterioration of the lens proteins without showing signs of cataracts until older age. alpha-crystallins not only possess chaperone-like activity in vitro, but can also remodel and protect the cytoskeleton, inhibit apoptosis, and enhance the resistance of cells to stress. Recent advances in the field of structure-function relationships of alpha-crystallins have provided the first clues to their underlying roles in tissues outside the lens. Proteins of the betagamma-crystallin family have been suggested to affect lens development, and are also expressed in tissues outside the lens. The goal of this paper is to highlight recent work with lens epithelial cells from alphaA- and alphaB-crystallin knockout mice. The use of lens epithelial cells suggests that crystallins have important cellular functions in the lens epithelium and not just the lens fiber cells as previously thought. These studies may be directly relevant to understanding the general cellular functions of crystallins.

Crystallins and hereditary cataracts: molecular mechanisms and potential for therapy

Hereditary childhood cataracts can arise from single-point mutations in genes encoding crystallins, the major protein components of the lens. The cataracts are most commonly inherited by an autosomal dominant mechanism. The nature of the changes in the lens resulting from these point mutations in crystallin genes has not been fully characterised. While aggregation and light scattering associated with expression of the mutant crystallin protein may be an end point, it is also necessary to determine the progression of changes induced at the level of development and differentiation. A key finding in recent work is that cell death or cytotoxicity is associated with mutations in αA-crystallin. The variable morphology or localisation of the cataract in different pedigrees, even with the identical crystallin gene mutation, has led to the idea that other environmental or genetic factors interact to give the final lens phenotype. The study of mechanisms of formation of hereditary cataracts may lead to a greater understanding of the mechanisms that lead to age-related cataracts, a very common cause of blindness in the ageing population.

Alpha‐crystallin expression affects microtubule assembly and prevents their aggregation

The molecular chaperones alphaA- and alphaB-crystallins are important for cell survival and genomic stability and associate with the tubulin cytoskeleton. The mitotic spindle is abnormally assembled in a number of alphaA-/- and alphaB-/- lens epithelial cells. However, no report to date has studied the effect of alpha-crystallin expression on tubulin/microtubule assembly in lens epithelial cells. In the current work we tested the hypothesis that the absence of alphaA- and alphaB-crystallins alters microtubule assembly. Microtubules were reconstituted from freshly dissected explants of wild-type, alphaA-/-, alphaB-/-, and alpha(A/B) -/- (DKO) mouse lens epithelia and examined by electron microscopic and biochemical analyses. The wild-type microtubules were 4 mum long and approximately 25 nm wide and had a characteristic protofilament structure, but alphaB-/- microtubules were 2.5-fold longer. Microtubule-associated proteins (MAPs) extracted from microtubules by washing with salt included transketolase, alpha-enolase, and betaB2-crystallin. In DKO lens epithelial microtubules but not in wild-type, alphaA-/- or alphaB-/- microtubules, extraction of the MAPs gave very long (14-20 microm) "polyfilament" assemblies that were tightly bundled. Addition of exogenous alpha-crystallin (alphaA+ alphaB) was ineffective in preventing polyfilament formation. However, normal microtubule structure could be restored by including MAPs derived from wild-type lens epithelial cells during microtubule reconstitution. Intriguingly, these data suggest that alpha-crystallin may interact with MAPs to inhibit aggregation of microtubules in lens epithelial cells. Sedimentation analysis and 90 degrees light scattering measurements showed that alpha-crystallin suppressed tubulin assembly in vitro. Alpha-crystallin did not have a strong effect on the GTPase activity of purified tubulin. SDS-PAGE analysis showed that alpha-crystallin prevented heat-induced aggregation of tubulin, suggesting that alpha-crystallin may affect microtubule assembly by maintaining the pool of unassembled tubulin.

Peroxide resistance in human and mouse lens epithelial cell lines is related to long-term changes in cell biology and architecture

It is well established that the response of the cell to environmental stress is a major basis for cell modification. Such modification is believed to adapt the cell to better survive its environment. Oxidative stress, a major and ubiquitous stressing factor, was selected for investigating the cellular response to stress. Most studies investigating such cellular response have employed examination of the cell either during or shortly after exposure to stress. We have employed a different approach arguing that the short-term response to stress obscures the biological changes that allow the cell to continue to thrive in its new environment. Reflecting this concept, murine and human cell lines capable of surviving regular exposure to toxic levels of H(2)O(2) or TBOOH have been developed. It was found that certain fundamental long-term changes in cell biology had occurred. The peroxide-resistant cells are diploid rather than aneuploid, show fundamental changes in the cytoskeletal cellular structure, suggesting less rigid more flexible cells, express a new lower molecular mass of p53, a key stress protein responder involved in adaptation, and finally have an immunochemical modification in alphaA-crystallin, a small heat-shock protein. Previously, it was found that there is a dramatic increase in catalase and gluthathione S-transferase activity and a remarkable limited change in expression in other antioxidative genes in these cells. The impact of these changes is discussed. It is apparent that evolutionary cell modifications can occur in response to relatively rapid changes in environment over periods ranging from days to months rather than the thousands of years considered in most evolutionary modifications.

Mimicking phosphorylation of the small heat-shock protein alphaB-crystallin recruits the F-box protein FBX4 to nuclear SC35 speckles

The mammalian small heat shock protein alphaB-crystallin can be phosphorylated at three different sites, Ser19, Ser45 and Ser59. We compared the intracellular distribution of wild-type, nonphosphorylatable and all possible pseudophosphorylation mutants of alphaB-crystallin by immunoblot and immunocytochemical analyses of stable and transiently transfected cells. We observed that pseudophosphorylation at two (especially S19D/S45D) or all three (S19D/S45D/S59D) sites induced the partial translocation of alphaB-crystallin from the detergent-soluble to the detergent-insoluble fraction. Double immunofluorescence studies showed that the pseudophosphorylation mutants localized in nuclear speckles containing the splicing factor SC35. The alphaB-crystallin mutants in these speckles were resistant to mild detergent treatment, and also to DNase I or RNase A digestion, indicating a stable interaction with one or more speckle proteins, not dependent on intact DNA or RNA. We further found that FBX4, an adaptor protein of the ubiquitin-protein isopeptide ligase SKP1/CUL1/F-box known to interact with pseudophosphorylated alphaB-crystallin, was also recruited to SC35 speckles when cotransfected with the pseudophosphorylation mutants. Because SC35 speckles also react with an antibody against alphaB-crystallin endogenously phosphorylated at Ser45, our findings suggest that alphaB-crystallin has a phosphorylation-dependent role in the ubiquitination of a component of SC35 speckles.

Identification of genes responsive to UV-A radiation in human lens epithelial cells using complementary DNA microarrays

UV-A radiation produces cataract in animals, enhances photoaging of the lens and skin and increases the phototoxicity of drugs. However, the nature of genes that are activated or repressed after cellular exposure to UV-A radiation remains enigmatic. Because lens epithelial cells exposed to UV-A radiation undergo apoptosis 4 h after exposure to the stress, we sought to establish the change in gene expression in cells by UV-A radiation using gene expression profiling using complementary DNA microarrays containing about 12 000 genes. We identified 78 genes abnormally expressed in UV-A-irradiated cells (showing >2.5-fold change at P < 0.05). These genes are implicated in various biological processes, including signal transduction and nucleic acid binding, and genes encoding enzymes. A majority of the genes were downregulated. Our analysis revealed that the expression of genes for the transcription factors ATF-3 and Pilot increased four-fold, whereas the gene for the apoptosis regulator NAPOR-1 decreased five-fold. These changes were confirmed by real-time quantitative reverse transcriptase-polymerase chain reaction. The calpain large polypeptide 3 (CANP3) gene also increased nine-fold after UV-A radiation. In addition, peroxisomal biogenesis factor 7, glucocorticoid receptor-alpha and tumor-associated calcium signal transducer genes decreased three- to eight-fold. Western blot analysis further confirmed the increase in protein expression of ATF-3 and CANP3 and decreased expression of glucocorticoid receptor-alpha in the irradiated cells. Surprisingly, most of these genes had not been previously shown to be modulated by UV-A radiation. Our results show that human lens epithelial cells respond to a single dose of UV-A radiation by enhancing or suppressing functionally similar sets of genes, some of which have opposing functions, around the time at which apoptosis occurs. These studies support the intriguing concept that activation of competing pathways favoring either cell survival or death is a means to coordinate the response of cells to UV-A stress.

A comparative analysis of αA- and αB-crystallin expression during the cell cycle in primary mouse lens epithelial cultures

AlphaA- and alphaB-crystallins are small heat shock proteins and molecular chaperones that prevent non-specific aggregation of denaturing proteins. Previous work in our laboratory has shown that lens epithelial cells derived from alphaA-/- mice exhibit slower growth, whereas alphaB-/- lens epithelial cells hyperproliferate at a higher rate in culture [Andley et al., J. Biol. Chem. 273 (1998) 31252; FASEB J. 15 (2001) 221]. Although both have been implicated in apoptosis and cell proliferation, direct analysis of their expression during the cell cycle has not been investigated. This study was undertaken to define the expression levels of alphaA and alphaB-crystallins during the cell cycle. Primary lens epithelial cell cultures derived from wild type mice were synchronized by serum starvation, and pulsed with bromodeoxyuridine (BrdU) at different times after re-stimulation with serum. Dual parameter flow cytometric studies with BrdU and propidium iodide (PI)-labeled cells were performed. Cells entered S phase 14 hr after serum re-stimulation. The duration of the S phase was 6 hr, and the total cell cycle transit time was between 24-27 hr. Enhanced expression of cyclin A, a protein essential for DNA synthesis was used as an additional marker to define the initiation of the S phase. Immunoblotting analysis demonstrated that the expression of alphaA and alphaB-crystallin was up to 10-fold higher in cells synchronized in G0 phase than in G1 phase. The levels of the proteins increased three-fold again as the cells entered the S phase and progressed to mitosis, but did not rise to the levels observed in G0 phase. This increase in expression of alphaA-crystallin resulted in part from enhanced synthesis during the S phase, as shown by an increase in [35S]methionine-labeling and immunoprecipitation of the radiolabeled alphaA-crystallin. The results were further confirmed by flow cytometric analysis using DNA content and alphaA-crystallin expression. The increase in alphaB-crystallin in S phase was paralleled by an increase in gene expression as shown by real-time RT-PCR analysis. These results demonstrate for the first time that in lens epithelial cells, alphaA and alphaB-crystallin levels are modulated during the cell cycle. Since the absence of alphaA and alphaB- crystallin in lens epithelial cells has been associated with disturbance of the tubulin cytoskeleton during mitosis, and with increased cell death or genomic instability, our results indicating that the alphaA- and alphaB-crystallin expression increases prior to mitosis are significant. The differential expression of these crystallins in the cell cycle may be important for optimal lens epithelial growth and lens transparency.

Cell kinetic status of mouse lens epithelial cells lacking αA- and αB-crystallin

alphaA- and alphaB-crystallins are small heat shock proteins and molecular chaperones that are known to prevent non-specific aggregation of denaturing proteins. Recent work indicates that alphaA-/- lens epithelial cells grow at a slower rate than wild-type cells, and cultured alphaB-/- cells demonstrate increased hyperproliferation and genomic instability, suggesting that these proteins may exert a direct effect on the cell cycle kinetics, and influence cell proliferation. However, the cell cycle parameters of alphaA/alphaBKO (double knockout) cells have not been analyzed. Here we investigate the cell cycle kinetics of synchronized mouse lens epithelial cultures derived from wild-type and alphaA/alphaB double knockout (alphaA/alphaBKO) mice using BrdU labeling of proliferating cells, and flow cytometric analysis. We also provide data on the changing pattern of expression of HSP25, a small heat shock protein in alphaA/alphaBKO and wild-type cells during the cell cycle. Using serum starvation to synchronize cells in the quiescent G0 phase, and restimulation with serum followed by BrdU labeling and flow cytometry, the data indicated that as compared to wild-type cells, a <50% smaller fraction of the alphaA/alphaBKO cells entered the DNA synthetic S phase of the cell cycle. Furthermore, there was a delay in cell cycle transit through S phase in alphaA/alphaBKO cells, suggesting that although capable of entering S phase, the alphaA/alphaBKO cells are blocked in G1 phase, and are delayed in their cell cycle progression. Immunoblot analysis with antibodies to the small heat shock protein HSP25 indicated that although HSP25 increased in G1 phase of wild-type cells, and remained elevated on further progression through the cell cycle, HSP25 accumulation was delayed to S phase in alphaA/alphaBKO cells. These data can be interpreted to indicate that mouse lens epithelial cell progression through the cell cycle is significantly affected by expression of alphaA and alphaB-crystallin.

Cell death triggered by a novel mutation in the alphaA-crystallin gene underlies autosomal dominant cataract linked to chromosome 21q

Hereditary cataract is a clinically and genetically heterogeneous lens disease that accounts for a significant proportion of visual impairment and blindness in childhood. The alphaA-crystallin (CRYAA) gene (CRYAA) encodes a member of the small-heat-shock protein (sHSP) family of molecular chaperones and is primarily and abundantly expressed in the ocular lens. Here, we have used linkage analysis to identify a novel missense mutation in CRYAA that underlies an autosomal dominant form of 'nuclear' cataract segregating in a four-generation Caucasian family. A maximum two-point LOD score (Z(max)) of 2.19 (maximum recombination fraction, theta(max)=0) and multipoint Z(max) of 3.3 (theta(max)=0) was obtained at marker D21S1885. Haplotype analysis indicated that the disease gene lay in the approximately 2.7 Mb physical interval between D21S1912 and D21S1260 flanking CRYAA on 21q22.3. Sequence analysis identified a C --> T transition in exon 1 of CRYAA from affected individuals that was predicted to result in the nonconservative substitution of cysteine for arginine at codon 49 (R49C). Transfection studies of lens epithelial cells revealed that, unlike wild-type CRYAA, the R49C mutant protein was abnormally localized to the nucleus and failed to protect from staurosporine-induced apoptotic cell death. This study has identified the first dominant cataract mutation in CRYAA located outside the phylogenetically conserved 'alpha-crystallin core domain' of the sHSP family.

A missense mutation in the gammaD crystallin gene (CRYGD) associated with autosomal dominant "coral-like" cataract linked to chromosome 2q

PURPOSE

Hereditary cataract is a clinically and genetically heterogeneous lens disorder that usually presents as a sight-threatening trait in childhood. The purpose of this study was to map and identify the mutation underlying an autosomal dominant form of coral-shaped cataract segregating in a three generation Caucasian pedigree.

METHODS

Genomic DNA was prepared from blood leucocytes, genotyping was performed using microsatellite markers, and LOD scores were calculated using the LINKAGE programs. Mutation detection was performed using direct sequencing and primer extension analysis. Following site-directed mutagenesis, mutant and wild type expression constructs were transfected into a human lens epithelial cell line (HLE B-3) and recombinant protein was detected by immunoblotting, imunofluorescence, and immunogold microscopy. Cell death was monitored by fluorescence activated cell sorting.

RESULTS

Significant evidence of linkage was detected at markers D2S371 (LOD score [Z]=3.81, recombination fraction [theta]=0) and D2S369 (Z=3.64, theta=0). Haplotyping indicated that the disease gene lay in the approximate 10 Mb physical interval between D2S1384 and D2S128, containing the gamma-crystallin gene (CRYGA-CRYGD) cluster on chromosome 2q33.3-q34. Sequencing of the CRYGA-CRYGD cluster identified a C->A transversion in exon 2 of CRYGD that was predicted to result in the non-conservative substitution of threonine for proline at amino-acid residue 23 (P23T) in the processed CRYGD protein. Transfection studies suggested that the P23T mutant was less soluble than its wild type counterpart when expressed in HLE B-3 cells.

CONCLUSIONS

This study has identified an eighth type of cataract morphology associated with CRYGD and suggests that a CRYGD mutation may underlie the historically important "coralliform" cataract first reported in 1895.

Phototoxicity in Human Lens Epithelial Cells Promoted by St. John's Wort¶

St. John's Wort (SJW), an over-the-counter antidepressant, contains hypericin, which absorbs light in the UV and visible ranges and is phototoxic to skin. To determine if it also could be phototoxic to the eye, we exposed human lens epithelial cells to 0.1-10 microM hypericin and irradiated them with 4 J/cm2 UV-A or 0.9 J/cm2 visible light. Neither hypericin exposure alone nor light exposure alone reduced cell viability. In contrast, cells exposed to hypericin in combination with UV-A or visible light underwent necrosis and apoptosis. The ocular antioxidants lutein and N-acetyl cysteine did not prevent damage. Thus, ingested SJW is potentially phototoxic to the eye and could contribute to early cataractogenesis. Precautions should be taken to protect the eye from intense sunlight while taking SJW.

Proteome analysis of lens epithelia, fibers, and the HLE B-3 cell line

PURPOSE

The purpose of this study is to compare the protein composition of the B-3 line of transformed human lens epithelial (HLE) cells to that of freshly dissected HLE cells. This provides baseline data on lens cell proteins from fresh lens cells and from the B-3 cell line, which is often used as a model system for the lens.

METHODS

Human lens epithelial cells adherent to the lens capsule were dissected into central (undifferentiated) and peripheral (partially differentiated) populations. Fully differentiated human lens fiber cells were isolated from the outer cortical layers of the lens. HLE B-3 cells were analyzed at several passage levels. Extracts were prepared from each cell type and the proteins resolved by two-dimensional polyacrylamide gel electrophoresis (2-DE). Representative gel patterns were visually compared, spots excised, and trypsin digests prepared. The peptide compositions of the digests were analyzed using either liquid chromatography electrospray ionization tandem mass spectrometry or atmospheric pressure-matrix-assisted laser desorption ionization mass spectrometry, using a liquid chromatography classic ion trap (LCQ) mass spectrometer.

RESULTS

Two-DE patterns were obtained for fresh and cultured cell types. Similar patterns were observed between central and peripheral HLE cells, both of which contained high levels of alphaA-, alphaB-, and betaB2-crystallins; alpha-enolase; and aldehyde dehydrogenase. HLE B-3 cultured cells were characterized by a marked loss of crystallins and a relatively higher level of noncrystallin proteins--most notably, high molecular weight, acidic proteins. Whereas subunit d of adenosine triphosphate (ATP) synthase, alphaB-crystallin, galectin, glyceraldehyde-3-phosphate dehydrogenase, alpha-enolase, actin, peptidylprolyl isomerase A, phosphatidylethanolamine-binding protein, and vimentin were present in both fresh and cultured lens epithelium, only the high abundance of alpha-enolase, galectin-1, and vimentin suggested that B-3 cells were lens derived.

CONCLUSIONS

Freshly dissected noncultured HLE cells from both central and peripheral regions contain a high concentration of crystallins that mask the detection of less abundant proteins by 2-DE. Transformation and culture of HLE cells causes a loss of these crystallins and an increase in the relative concentration of other proteins. However, most of these noncrystallin proteins were different from those observed in noncultured HLE cells. These results suggest that transformation markedly alters the protein expression pattern in immortalized HLE cells and that caution should be exercised when using them to study properties of HLE cells in vivo.

Hyperproliferation and p53 status of lens epithelial cells derived from alphaB-crystallin knockout mice

alphaB-Crystallin, a major protein of lens fiber cells, is a stress-induced chaperone expressed at low levels in the lens epithelium and numerous other tissues, and its expression is enhanced in certain pathological conditions. However, the function of alphaB in these tissues is not known. Lenses of alphaB-/- mice develop degeneration of specific skeletal muscles but do not develop cataracts. Recent work in our laboratory indicates that primary cultures of alphaB-/- lens epithelial cells demonstrate genomic instability and undergo hyperproliferation at a frequency 4 orders of magnitude greater than that predicted by spontaneous immortalization of rodent cells. We now demonstrate that the hyperproliferative alphaB-/- lens epithelial cells undergo phenotypic changes that include the appearance of the p53 protein as shown by immunoblot analysis. Sequence analysis showed a lack of mutations in the p53 coding region of hyperproliferative alphaB-/- cells. However, the reentry of hyperproliferative alphaB-/- cells into S phase and mitosis after DNA damage by gamma-irradiation were consistent with impaired p53 checkpoint function in these cells. The results demonstrate that expression of functionally impaired p53 is one of the factors that promote immortalization of lens epithelial cells derived from alphaB-/- mice. Fluorescence in situ hybridization using probes prepared from centromere-specific mouse P1 clones of chromosomes 1 and 9 demonstrated that the hyperproliferative alphaB-/- cells were 30% diploid and 70% tetraploid, whereas wild type cells were 83% diploid. Further evidence of genomic instability was obtained when the hyperproliferative alphaB-/- cells were labeled with anti-beta-tubulin antibodies. Examination of the hyperproliferative alphaB-/- mitotic profiles revealed the presence of cells that failed to round up for mitosis, or arrested in cytokinesis, and binucleated cells in which nuclear division had occurred without cell division. These results suggest that the stress protein and molecular chaperone alphaB-crystallin protects cells from acquiring impaired p53 protein and genomic instability.

A comprehensive analysis of the expression of crystallins in mouse retina

PURPOSE

Crystallins are expressed at high levels in lens fiber cells. Recent studies have revealed that several members of the alpha, beta, and gamma-crystallin family are also distributed in many non-lens tissues, though at lower levels. We observed that the use of retinal RNA as target for both custom I-Gene microarrays and Affymetrix GeneChips revealed significant expression of many crystallin genes. This prompted us to undertake a comprehensive investigation to delineate the baseline expression of crystallin genes in the adult mouse retina.

METHODS

Quantitative RT-PCR was carried out using gene specific primers (derived from the mouse genomic sequence) for each crystallin gene. Immunofluorescence studies using frozen sections of the mouse retinas were performed with crystallin-specific antibodies. Retinal lysates were analyzed by immunoblotting using antibodies specific to alphaA and alphaB crystallins and those produced against total beta-crystallin and gamma-crystallin fractions of bovine lenses.

RESULTS

Microarray analysis followed by quantitative RT-PCR revealed that mouse retinal cells express transcripts for 20 different members of the crystallin gene family; these are alphaA, alphaA-INS, alphaA-nov1, alphaB, betaA1, betaA2, betaA3, betaA4, betaB1, betaB2, betaB3, gammaA, gammaC, gammaD, gammaE, gammaF, gammaS, mu, zeta, and lambda-crystallin. The gene products of alphaA, alphaB, beta-, and gamma-crystallins are detected in the outer and inner nuclear layers of the retina by immunofluorescence analysis. In addition, alpha and beta-crystallins are detected in the photoreceptor inner segments. Retinal expression of these proteins was further confirmed by immunoblot analysis. Interestingly, our studies also showed a significant animal-to-animal variation in the expression level of some of the crystallins.

CONCLUSIONS

Our results establish the expression of many crystallins in the adult mouse retina. Detection of crystallins in the retinal nuclear layers, though surprising, is consistent with their proposed role in cell survival and genomic stability. We suggest that crystallins play vital functions in protecting retinal neurons from damage by environmental and/or metabolic stress.

Metabolism of lipid derived aldehyde, 4-hydroxynonenal in human lens epithelial cells and rat lens

PURPOSE

An earlier study showed that 4-hydroxynonenal (HNE), formed as a result of increased lipid peroxidation in oxidative stress, causes loss of lens transparency. To determine how HNE is detoxified in ocular tissues, its metabolism in cultured human lens epithelial cells (HLECs) as well as rat lens was investigated.

METHODS

Rat lens or HLECs were incubated with 30 nmol (5 x 10(5) cpm/ micromol) of HNE in 2 mL Krebs-Hansleit buffer for 1 hour at 37 degrees C. The medium, after ultrafiltration was analyzed by high performance liquid chromatography (HPLC), using a C-18 reversed-phase column. The metabolites were separated by using a gradient consisting of solvent A (0.1% aqueous trifluoroacetic acid) and solvent B (100% acetonitrile) at a flow rate of 1 mL/min. Fractions containing radioactivity were pooled and analyzed using electrospray ionization mass spectroscopy (ESI-MS) or gas chromatography-chemical ionization mass spectroscopy (GC/CI-MS).

RESULTS

On HPLC, the incubation media from cultured lens and HLECs separated into three major radioactive peaks. Peak I of the HLECs and lens treated with HNE was identified to be a mixture of glutathione (GS) conjugates of HNE and 1,4-dihydroxy-2-nonene (DHN). The identity of the conjugates was confirmed by ESI-MS. Based on the retention times, peaks II, and III were assigned to 4-hydroxy-2-nonenoic acid (HNA) and unmetabolized HNE, respectively. The identities of HNA and HNE were confirmed by spiking the tissue extracts with synthetic metabolites and finally by GC/CI-MS. Sorbinil, an aldose reductase (AR) inhibitor, attenuated GS-DHN levels and cyanamide, an aldehyde dehydrogenase inhibitor, decreased formation of HNA.

CONCLUSIONS

The results show that the major metabolic transformation of HNE in rat lens and HLECs involves conjugation with GS and oxidation to HNA. The GS-HNE conjugate is reduced to GS-DHN by AR. Thus, under normal physiological conditions, the lens has multiple routes to detoxify HNE. However, oxidative stress may overwhelm the metabolic capacity of the lens to detoxify HNE and lead to formation of cataract.