S-methylated cysteines in human lens gamma S-crystallins

New insights into change of lens proteins' stability with ageing under physiological conditions

BACKGROUND

Age-related cataract, which presents as a cloudy lens, is the primary cause of vision impairment worldwide and can cause more than 80% senile blindness. Previous studies mainly explored the profile of lens proteins at a low concentration because of technical limitations, which could not reflect physiological status. This study focuses on protein stability changes with ageing under physiological conditions using a novel equipment, Unchained Labs (Uncle), to evaluate protein thermal stability.

METHODS

Samples were assessed through Unchained Labs, size-exclusion chromatography, western blot and biophysics approaches including the Thioflavin T, ultraviolet and internal fluorescence.

RESULTS

With age, the melting temperature value shifted from 67.8°C in the young group to 64.2°C in the aged group. Meanwhile, crystallin may form more isomeric oligomers and easy to be degraded in aged lenses. The spectroscopic and size-exclusion chromatography results show a higher solubility after administrated with lanosterol under the environmental stress.

CONCLUSION

We are the first to explore rabbit lens protein stability changes with ageing using biophysical methods under physiological conditions, and this study can conclude that the structural stability and solubility of lens proteins decrease with ageing. Additionally, lanosterol could aid in resolving protein aggregation, making it a potential therapeutic option for cataracts. So, this study provides cataract models for anti-cataract drug developments.

The Structure and Stability of the Disulfide-Linked γS-Crystallin Dimer Provide Insight into Oxidation Products Associated with Lens Cataract Formation

The reducing environment in the eye lens diminishes with age, leading to significant oxidative stress. Oxidation of lens crystallin proteins is the major contributor to their destabilization and deleterious aggregation that scatters visible light, obscures vision, and ultimately leads to cataract. However, the molecular basis for oxidation-induced aggregation is unknown. Using X-ray crystallography and small-angle X-ray scattering, we describe the structure of a disulfide-linked dimer of human γS-crystallin that was obtained via oxidation of C24. The γS-crystallin dimer is stable at glutathione concentrations comparable to those in aged and cataractous lenses. Moreover, dimerization of γS-crystallin significantly increases the protein's propensity to form large insoluble aggregates owing to non-cooperative domain unfolding, as is observed in crystallin variants associated with early-onset cataract. These findings provide insight into how oxidative modification of crystallins contributes to cataract and imply that early-onset and age-related forms of the disease share comparable development pathways.

Dynamic disulfide exchange in a crystallin protein in the human eye lens promotes cataract-associated aggregation

Increased light scattering in the eye lens due to aggregation of the long-lived lens proteins, crystallins, is the cause of cataract disease. Several mutations in the gene encoding human γD-crystallin (HγD) cause misfolding and aggregation. Cataract-associated substitutions at Trp⁴² cause the protein to aggregate in vitro from a partially unfolded intermediate locked by an internal disulfide bridge, and proteomic evidence suggests a similar aggregation precursor is involved in age-onset cataract. Surprisingly, WT HγD can promote aggregation of the W42Q variant while itself remaining soluble. Here, a search for a biochemical mechanism for this interaction has revealed a previously unknown oxidoreductase activity in HγD. Using in vitro oxidation, mutational analysis, cysteine labeling, and MS, we have assigned this activity to a redox-active internal disulfide bond that is dynamically exchanged among HγD molecules. The W42Q variant acts as a disulfide sink, reducing oxidized WT and forming a distinct internal disulfide that kinetically traps the aggregation-prone intermediate. Our findings suggest a redox "hot potato" competition among WT and mutant or modified polypeptides wherein variants with the lowest kinetic stability are trapped in aggregation-prone intermediate states upon accepting disulfides from more stable variants. Such reactions may occur in other long-lived proteins that function in oxidizing environments. In these cases, aggregation may be forestalled by inhibiting disulfide flow toward mutant or damaged polypeptides.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

Isomerization of Asp residues plays an important role in αA-crystallin dissociation

Aged cataract formation is caused by the accumulative precipitation of lens proteins incorporating diverse post-translational modifications. α-Crystallin, a major structural and functional lens protein, consists of a large polymeric structure that is dissociated and insolubilized with accumulative post-translational modifications. One such modification, isomerization of Asp, was recently identified in αB-crystallin monomers derived from aged lens. However, the distributions of Asp isomers in each lens fraction remain unknown. Here, α-crystallin fractions from aged lens were separated into heteropolymeric and monomeric forms to determine the Asp isomerization ratios in each fraction. Lens of four different ages were homogenized and centrifuged, and the soluble fraction was applied to size-exclusion chromatography. The heteropolymeric α-crystallin and monomeric crystallin fractions were obtained and concentrated. After trypsin digestion, each fraction was independently applied to liquid chromatography equipped with mass spectrometry to extract α-crystallin-derived peptides containing Asp isomers. The results showed that Asp58, Asp84 and Asp151 of αA-crystallin were highly isomerized in the monomeric fraction, but not isomerized to the same level in the heteropolymeric fraction. Each type of Asp isomerization increased in an age-dependent manner, was site-specific and was similar to previous results from lens water-insoluble fractions. These results imply that isomerization of Asp residues leads to dissociation of αA-crystallin from the heteropolymeric state and induces insolubilization in aged lens. Taken together, our findings suggest that isomerization of Asp might disrupt the higher order polymeric state of α-crystallin, resulting in decreased solubility and function, ultimately contributing to lens protein impairment and cataract formation with aging.

The etiology of human age-related cataract. Proteins don't last forever

BACKGROUND

It is probable that the great majority of human cataract results from the spontaneous decomposition of long-lived macromolecules in the human lens. Breakdown/reaction of long-lived proteins is of primary importance and recent proteomic analysis has enabled the identification of the particular crystallins, and their exact sites of amino acid modification.

SCOPE OF REVIEW

Analysis of proteins from cataractous lenses revealed that there are sites on some structural proteins that show a consistently greater degree of deterioration than age-matched normal lenses.

MAJOR CONCLUSIONS

The most abundant posttranslational modification of aged lens proteins is racemization. Deamidation, truncation and crosslinking, each arising from the spontaneous breakdown of susceptible amino acids within proteins, are also present. Fundamental to an understanding of nuclear cataract etiology, it is proposed that once a certain degree of modification at key sites occurs, that protein-protein interactions are disrupted and lens opacification ensues.

GENERAL SIGNIFICANCE

Since long-lived proteins are now recognized to be present in many other sites of the body, such as the brain, the information gleaned from detailed analyses of degraded proteins from aged lenses will apply more widely to other age-related human diseases. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Biophysical chemistry of the ageing eye lens

This review examines both recent and historical literature related to the biophysical chemistry of the proteins in the ageing eye, with a particular focus on cataract development. The lens is a vital component of the eye, acting as an optical focusing device to form clear images on the retina. The lens maintains the necessary high transparency and refractive index by expressing crystallin proteins in high concentration and eliminating all large cellular structures that may cause light scattering. This has the consequence of eliminating lens fibre cell metabolism and results in mature lens fibre cells having no mechanism for protein expression and a complete absence of protein recycling or turnover. As a result, the crystallins are some of the oldest proteins in the human body. Lack of protein repair or recycling means the lens tends to accumulate damage with age in the form of protein post-translational modifications. The crystallins can be subject to a wide range of age-related changes, including isomerisation, deamidation and racemisation. Many of these modification are highly correlated with cataract formation and represent a biochemical mechanism for age-related blindness.

Electrostatic origin of in vitro aggregation of human γ-crystallin

The proteins α-, β-, and γ-crystallins are the major components of the lens in the human eye. Using dynamic light scattering method, we have performed in vitro investigations of protein-protein interactions in dilute solutions of human γ-crystallin and α-crystallin. We find that γ-crystallin spontaneously aggregates into finite-sized clusters in phosphate buffer solutions. There are two distinct populations of unaggregated and aggregated γ-crystallins in these solutions. On the other hand, α-crystallin molecules are not aggregated into large clusters in solutions of α-crystallin alone. When α-crystallin and γ-crystallin are mixed in phosphate buffer solutions, we demonstrate that the clusters of γ-crystallin are prevented. By further investigating the roles of temperature, protein concentration, pH, salt concentration, and a reducing agent, we show that the aggregation of γ-crystallin under our in vitro conditions arises from non-covalent electrostatic interactions. In addition, we show that aggregation of γ-crystallin occurs under the dilute in vitro conditions even in the absence of oxidizing agents that can induce disulfide cross-links, long considered to be responsible for human cataracts. Aggregation of γ-crystallin when maintained under reducing conditions suggests that oxidation does not contribute to the aggregation in dilute solutions.

Old proteins and the Achilles heel of mass spectrometry. The role of proteomics in the etiology of human cataract

Proteomics may have enabled the root cause of a major human-blinding condition, age-related cataract, to be established. Cataract appears to result from the spontaneous decomposition of long-lived macromolecules in the human lens, and recent proteomic analysis has enabled both the particular crystallins, and the specific sites of amino acid modification within each polypeptide, to be identified. Analysis of proteins from cataract lenses has demonstrated that there are key sites on some structural proteins that show a consistently greater degree of deterioration than age-matched normal lenses. Proteomic analysis, using MS, revealed that the most abundant posttranslational modification of aged lens proteins is racemization. This is somewhat ironic, since structural isomers can be viewed as the "Achilles heel" of MS and there are typically few, if any, differences in the MS/MS spectra of tryptic peptides containing one d-amino acid. It is proposed that once a certain level of spontaneous PTM at key sites occurs, that protein-protein interactions are disrupted, and binding of complexes to cell membranes takes place that impairs cell-to-cell communication. These findings may apply more widely to age-related human diseases, in particular where the deterioration of long-lived proteins is a crucial component in the etiology.

Automethylation activities within the mixed lineage leukemia-1 (MLL1) core complex reveal evidence supporting a "two-active site" model for multiple histone H3 lysine 4 methylation

The mixed lineage leukemia-1 (MLL1) core complex predominantly catalyzes mono- and dimethylation of histone H3 at lysine 4 (H3K4) and is frequently altered in aggressive acute leukemias. The molecular mechanisms that account for conversion of mono- to dimethyl H3K4 (H3K4me1,2) are not well understood. In this investigation, we report that the suppressor of variegation, enhancer of zeste, trithorax (SET) domains from human MLL1 and Drosophila Trithorax undergo robust intramolecular automethylation reactions at an evolutionarily conserved cysteine residue in the active site, which is inhibited by unmodified histone H3. The location of the automethylation in the SET-I subdomain indicates that the MLL1 SET domain possesses significantly more conformational plasticity in solution than suggested by its crystal structure. We also report that MLL1 methylates Ash2L in the absence of histone H3, but only when assembled within a complex with WDR5 and RbBP5, suggesting a restraint for the architectural arrangement of subunits within the complex. Using MLL1 and Ash2L automethylation reactions as probes for histone binding, we observed that both automethylation reactions are significantly inhibited by stoichiometric amounts of unmethylated histone H3, but not by histones previously mono-, di-, or trimethylated at H3K4. These results suggest that the H3K4me1 intermediate does not significantly bind to the MLL1 SET domain during the dimethylation reaction. Consistent with this hypothesis, we demonstrate that the MLL1 core complex assembled with a catalytically inactive SET domain variant preferentially catalyzes H3K4 dimethylation using the H3K4me1 substrate. Taken together, these results are consistent with a “two-active site” model for multiple H3K4 methylation by the MLL1 core complex.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10-50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Mass spectrometry-based proteomics approaches applied in cataract research

Cataract, the opacification of the eye lens, is the leading cause of blindness worldwide--it accounts for approximately 42% of all cases. The lens fibers have the highest protein content within the body, more than 35% of their wet weight. Given the eye lens pure composition of highly abundant structural proteins crystallins (up to 90%), it seems to be an ideal proteomic entity to study and might be also hypothesized to model the other protein conformational diseases. Crystallins are extremely long-lived, and there is virtually no protein turnover. This provides great opportunities for post-translational modifications (PTM) to occur and to predispose lens to the cataract formation. Despite recent progress in proteomics, the human lens proteome remains largely unknown. Mass spectrometry hold great promise to determine which crystallin modifications lead to a cataract. Quantitative analysis of PTMs at the peptide level with proteomics is a powerful bioanalytical tool for lens-tissue samples, and provides more comprehensive results. New mass spectrometry-based approaches that are being applied to lens research will be highlighted. Finally, the future directions of proteomics cataract research will be outlined.

alpha-Lipoic acid alters post-translational modifications and protects the chaperone activity of lens alpha-crystallin in naphthalene-induced cataract

PURPOSE

To evaluate whether alpha-lipoic acid (LA) inhibits lens opacity of naphthalene-induced cataract by altering post-translational modifications (PTMs) and protecting the chaperone activity of alpha-crystallins.

METHODS

Forty-five Sprague-Dawley rats were divided into three groups: control, naphthalene, and naphthalene plus LA. Cataracts were induced by oral administration of 1 g naphthalene/kg body weight/day. Rats in the naphthalene plus LA group were also fed 30 mg LA/day. The development of naphthalene-initiated cataract was monitored every week by slit lamp microscopy for nine weeks, then the lens proteins were separated by HPLC, and peaks corresponding to alpha-crystallins were resolved on 2-DE. The spots of 2-DE were subjected to mass spectrometry to identify PTMs. Chaperone activity of alpha-crystallins was measured by heat-induced aggregation of betaL-crystallin.

RESULTS

The lenses of rats fed with naphthalene plus LA exhibited less light scattering than that fed with only naphthalene at three weeks after treatment (P < 0.01). C-terminal truncated alphaA crystallin was detected in naphthalene-induced cataract and was abrogated by LA treatment. Several other post-translational modifications were identified including methylation, phosphorylation, acetylation, carbamylation, and oxidation.

CONCLUSIONS

Our data are the first to show PTM changes induced by naphthalene in rat lenses. Our findings also indicate that LA can inhibit naphthalene-induced lens opacity by altering PTM and protecting the chaperone activity of alpha-crystallins.

Presbyopia and cataract: a question of heat and time

Not only are human lenses different in many ways from those of non-primates, they also undergo dramatic changes with age. These age-dependent alterations lead to perturbations in the properties of older lenses, and ultimately to disturbances in visual function, which typically become apparent at middle age. Recent data suggest that many, if not all, of these age-dependent features can be traced to the lack of macromolecular turnover in the lens and to the inexorable modifications to proteins and membrane components over a period of decades. Exposure of lenses to heat can reproduce many of these alterations, suggesting that long-term incubation at body temperature may be an important factor in aging the human lens. Two conclusions flow from this. Firstly, the human lens may be an ideal tissue for studying macromolecular aging in man. Secondly, it will be extremely challenging to examine the origin of human age-related conditions, such as presbyopia and nuclear cataract, using traditional laboratory animals. Characterising the unfolding and decomposition of long-lived macromolecules appears to provide the key to understanding the two most common human lens disorders: presbyopia and age-related nuclear cataract.

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Identification of the primary targets of carbamylation in bovine lens proteins by mass spectrometry

PURPOSE

Carbamylation, an important post-translational modification of proteins, inevitably causes conformational changes of lens proteins. It may increase aggregation between crystallin molecules and disrupt the close packing required for transparency thus leading to cataract. The aim of this study was to isolate the primary targets of carbamylation in the lens and identify them by mass spectrometry.

MATERIALS AND METHODS

Fresh intact bovine lenses were incubated with 100 mM potassium cyanate for 7 days. The proteins in the water-soluble fractions from the normal control and the cyanate-modified lens proteins were separated by two-dimensional (2-D) gel electrophoresis with identification after silver staining. Protein spots that differed between the normal and carbamylated groups were selected for further analysis using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

RESULTS

The 2-D gel results showed that the major lens proteins were in the section of pI 5-8, with relative molecular masses of 20-35 kDa, and changes in the carbamylated fraction like strings of beads indicating modification. The mass spectrometry analysis and a database search identified carbamylated proteins originating from alphaA-crystallin, betaB2- and gammaS-(betaS)-crystallins.

CONCLUSIONS

These crystallins may be vulnerable proteins targeted by carbamylation. The accumulated aggregation and loss of chaperone activity may contribute to cataract formation.

Proteomic analysis of the oxidation of cysteine residues in human age-related nuclear cataract lenses

Loss of protein thiols is a key feature associated with the onset of age-related nuclear cataract (ARNC), however, little is known about the specific sites of oxidation of the crystallins. We investigated cysteine residues in ARNC lenses and compared them with age-matched normal lenses. Proteomic analysis of tryptic digests revealed ten cysteine residues in older normal lenses that showed no significant oxidation compared to foetal counterparts (Cys 170 in betaA1/3-crystallin, Cys 32 in betaA4-crystallin, Cys 79 in betaB1-crystallin, Cys 22, Cys 78/79, C153 in gammaC-crystallin and Cys 22, Cys 24 and Cys 26 in gammaS-crystallin). Although these thiols were not oxidised in normal lenses past the 6th decade, they were present largely as disulphides in the ARNC lenses. By contrast, two cysteine residues, Cys 41 in gammaC-crystallin and Cys 18 in gammaD-crystallin, were not oxidised, even in advanced ARNC lenses. These cysteines are buried deep within the protein and any unfolding associated with cataract must be insufficient to expose them to the oxidative environment present in the centre of advanced ARNC lenses. The vast majority of the loss of protein thiol observed in such lenses is due to disulphide bond formation.

Deamidation alters the structure and decreases the stability of human lens betaA3-crystallin

According to the World Health Organization, cataracts account for half of the blindness in the world, with the majority occurring in developing countries. A cataract is a clouding of the lens of the eye due to light scattering of precipitated lens proteins or aberrant cellular debris. The major proteins in the lens are crystallins, and they are extensively deamidated during aging and cataracts. Deamidation has been detected at the domain and monomer interfaces of several crystallins during aging. The purpose of this study was to determine the effects of two potential deamidation sites at the predicted interface of the betaA3-crystallin dimer on its structure and stability. The glutamine residues at the reported in vivo deamidation sites of Q180 in the C-terminal domain and at the homologous site Q85 in the N-terminal domain were substituted with glutamic acid residues by site-directed mutagenesis. Far-UV and near-UV circular dichroism spectroscopy indicated that there were subtle differences in the secondary structure and more notable differences in the tertiary structure of the mutant proteins compared to that of the wild type betaA3-crystallin. The Q85E/Q180E mutant also was more susceptible to enzymatic digestion, suggesting increased solvent accessibility. These structural changes in the deamidated mutants led to decreased stability during unfolding in urea and increased precipitation during heat denaturation. When simulating deamidation at both residues, there was a further decrease in stability and loss of cooperativity. However, multiangle-light scattering and quasi-elastic light scattering experiments showed that dimer formation was not disrupted, nor did higher-order oligomers form. These results suggest that introducing charges at the predicted domain interface in the betaA3 homodimer may contribute to the insolubilization of lens crystallins or favor other, more stable, crystallin subunit interactions.

Age-related changes in human crystallins determined from comparative analysis of post-translational modifications in young and aged lens: does deamidation contribute to crystallin insolubility?

We have employed recently developed blind modification search techniques to generate the most comprehensive map of post-translational modifications (PTMs) in human lens constructed to date. Three aged lenses, two of which had moderate cataract, and one young control lens were analyzed using multidimensional liquid chromatography mass spectrometry. In total, 491 modification sites in lens proteins were identified. There were 155 in vivo PTM sites in crystallins: 77 previously reported sites and 78 newly detected PTM sites. Several of these sites had modifications previously undetected by mass spectrometry in lens including carboxymethyl lysine (+58 Da), carboxyethyl lysine (+72 Da), and an arginine modification of +55 Da with yet unknown chemical structure. These new modifications were observed in all three aged lenses but were not found in the young lens. Several new sites of cysteine methylation were identified indicating this modification is more extensive in lens than previously thought. The results were used to estimate the extent of modification at specific sites by spectral counting. We tested the long-standing hypothesis that PTMs contribute to age-related loss of crystallin solubility by comparing spectral counts between the water-soluble and water-insoluble fractions of the aged lenses and found that the extent of deamidation was significantly increased in the water-insoluble fractions. On the basis of spectral counting, the most abundant PTMs in aged lenses were deamidations and methylated cysteines with other PTMs present at lower levels.

Solution structure of (gamma)S-crystallin by molecular fragment replacement NMR

The solution structure of murine gammaS-crystallin (gammaS) has been determined by multidimensional triple resonance NMR spectroscopy, using restraints derived from two sets of dipolar couplings, recorded in different alignment media, and supplemented by a small number of NOE distance restraints. gammaS consists of two topologically similar domains, arranged with an approximate twofold symmetry, and each domain shows close structural homology to closely related (approximately 50% sequence identity) domains found in other members of the gamma-crystallin family. Each domain consists of two four-strand "Greek key" beta-sheets. Although the domains are tightly anchored to one another by the hydrophobic surfaces of the two inner Greek key motifs, the N-arm, the interdomain linker and several turn regions show unexpected flexibility and disorder in solution. This may contribute entropic stabilization to the protein in solution, but may also indicate nucleation sites for unfolding or other structural transitions. The method used for solving the gammaS structure relies on the recently introduced molecular fragment replacement method, which capitalizes on the large database of protein structures previously solved by X-ray crystallography and NMR.

Mass spectrometry in aging research

This review covers the application of mass spectrometric techniques to aging research. Modern proteomic strategies will be discussed as well as the targeted analysis of specific proteins for the correlation of post-translational modifications with protein function. Selected examples will show both the power and also current limitations of the respective techniques. Experimental results and strategies are discussed in view of current theories of the aging process.

Age-related nuclear cataract—oxidation is the key

Age is by far the biggest risk factor for cataract, and it is sometimes assumed that cataract is simply an amplification of this aging process. This appears not to be the case, since the lens changes associated with aging and cataract are distinct. Oxidation is the hallmark of age-related nuclear (ARN) cataract. Loss of protein sulfhydryl groups, and the oxidation of methionine residues, are progressive and increase as the cataract worsens until >90% of cysteine and half the methionine residues are oxidised in the most advanced form. By contrast, there may be no significant oxidation of proteins in the centre of the lens with advancing age, even past age 80. The key factor in preventing oxidation seems to be the concentration of nuclear glutathione (GSH). Provided that nuclear GSH levels can be maintained above 2 mm, it appears that significant protein oxidation and posttranslational modification by reactive small molecules, such as ascorbate or UV filter degradation products, is not observed. Adequate coupling of the metabolically-active cortex, the source of antioxidants such as GSH, to the quiescent nucleus, is crucial especially since it would appear that the cortex remains viable in old lenses, and even possibly in ARN cataract lenses. Therefore it is vital to understand the reason for the onset of the lens barrier. This barrier, which becomes apparent in middle age, acts to impede the flow of small molecules between the cortex and the nucleus. The barrier, rather than nuclear compaction (which is not observed in human lenses), may contribute to the lowered concentration of GSH in the lens nucleus after middle age. By extending the residence time within the lens centre, the barrier also facilitates the decomposition of intrinsically unstable metabolites and may exacerbate the formation of H(2)O(2) in the nucleus. This hypothesis, which is based on the generation of reactive oxygen species and reactive molecules within the nucleus itself, shifts the focus away from theories for cataract that postulated a primary role for oxidants generated outside of the lens. Unfortunately, due to marked variability in the lenses of different species, there appears at present to be no ideal animal model system for studying human ARN cataract.

Modifications of human betaA1/betaA3-crystallins include S-methylation, glutathiolation, and truncation

Disulfide bonding of lens crystallins contributes to the aggregation and insolubilization of these proteins that leads to cataract. A high concentration of reduced glutathione is believed to be key in preventing oxidation of crystallin sulfhydryls to form disulfide bonds. This protective role is decreased in aged lenses because of lower glutathione levels, especially in the nucleus. We recently found that human gamma-crystallins undergo S-methylation at exposed cysteine residues, a reaction that may prevent disulfide bonding. We report here that betaA1/A3-crystallins are also methylated at specific cysteine residues and are the most heavily methylated of the human lens crystallins. Among the methylated sites, Cys 64, Cys 99, and Cys 167 of betaA1-crystallin, methylation at Cys 99 is highest. Cys 64 and Cys 99 are also glutathiolated, even in a newborn lens. These post-translational modifications of the exposed cysteines may be important for maintaining the crystallin structure required for lens transparency. Previously unreported N-terminal truncations were also found.

Sulfur in human crystallins

Molecular models of human gamma-crystallins and the 'alpha-crystallin domain' of human alphaA-crystallin have been built based on available related X-ray crystal structures. The accessibilities of the component cysteine, methionine and tryptophan side chains in the crystallin models have been calculated. The reactivities of these cysteines, which are oxidised in cataract, are assessed based on their known modifications and within the context of their location within the 3D models.

Ageing and vision: structure, stability and function of lens crystallins

The alpha-, beta- and gamma-crystallins are the major protein components of the vertebrate eye lens, alpha-crystallin as a molecular chaperone as well as a structural protein, beta- and gamma-crystallins as structural proteins. For the lens to be able to retain life-long transparency in the absence of protein turnover, the crystallins must meet not only the requirement of solubility associated with high cellular concentration but that of longevity as well. For proteins, longevity is commonly assumed to be correlated with long-term retention of native structure, which in turn can be due to inherent thermodynamic stability, efficient capture and refolding of non-native protein by chaperones, or a combination of both. Understanding how the specific interactions that confer intrinsic stability of the protein fold are combined with the stabilizing effect of protein assembly, and how the non-specific interactions and associations of the assemblies enable the generation of highly concentrated solutions, is thus of importance to understand the loss of transparency of the lens with age. Post-translational modification can have a major effect on protein stability but an emerging theme of the few studies of the effect of post-translational modification of the crystallins is one of solubility and assembly. Here we review the structure, assembly, interactions, stability and post-translational modifications of the crystallins, not only in isolation but also as part of a multi-component system. The available data are discussed in the context of the establishment, the maintenance and finally, with age, the loss of transparency of the lens. Understanding the structural basis of protein stability and interactions in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

Methylation and carbamylation of human gamma-crystallins

Accessible sulfhydryls of cysteine residues are likely sites of reaction in long-lived proteins such as human lens crystallins. Disulfide bonding between cysteines is a major contributor to intermolecular cross-linking and aggregation of crystallins. A recently reported modification of gammaS-crystallins, S-methylation of cysteine residues, can prevent disulfide formation. The aim of this study was to determine whether cysteines in gammaC-, gammaD-, and gammaB-crystallins are also S-methylated. Our data show that all the gamma-crystallins are S-methylated, but only at specific cysteines. In gammaD-crystallin, methylation is exclusively at Cys 110, whereas in gammaC- and gammaB-crystallins, the principal methylation site is Cys 22 with minor methylation at Cys 79. gammaD-crystallin is the most heavily methylated gamma-crystallin. gammaD-Crystallins from adult lenses are 37%-70% methylated, whereas gammaC and gammaB are approximately 12% methylated. The specificity of gamma-crystallin methylation and its occurrence in young clear lenses supports the idea that inhibition of disulfide bonding by S-methylation may play a protective role against cataract. Another modification, not reported previously, is carbamylation of the N termini of gammaB-, gammaC-, gammaD-crystallins. N-terminal carbamylation is likely a developmentally related modification that does not negatively impact crystallin function.