Glutathione adducts, not carbamylated lysines, are the major modification of lens alpha-crystallins from renal failure patients

Truncation of alphaB-crystallin by the myopathy-causing Q151X mutation significantly destabilizes the protein leading to aggregate formation in transfected cells

Here we investigate the effects of a myopathy-causing mutation in alphaB-crystallin, Q151X, upon its structure and function. This mutation removes the C-terminal domain of alphaB-crystallin, which is expected to compromise both its oligomerization and chaperone activity. We compared this to two other alphaB-crystallin mutants (450delA, 464delCT) and also to a series of C-terminal truncations (E164X, E165X, K174X, and A171X). We find that the effects of the Q151X mutation were not always as predicted. Specifically, we have found that although the Q151X mutation decreased oligomerization of alphaB-crystallin and even increased some chaperone activities, it also significantly destabilized alphaB-crystallin causing it to self-aggregate. This conclusion was supported by our analyses of both the other disease-causing mutants and the series of C-terminal truncation constructs of alphaB-crystallin. The 450delA and 464delCT mutants could only be refolded and assayed as a complex with wild type alphaB-crystallin, which was not the case for Q151X alphaB-crystallin. From these studies, we conclude that all three disease-causing mutations (450delA, 464delCT, and Q151X) in the C-terminal extension destabilize alphaB-crystallin and increase its tendency to self-aggregate. We propose that it is this, rather than a catastrophic loss of chaperone activity, which is a major factor in the development of the reported diseases for the three disease-causing mutations studied here. In support of this hypothesis, we show that Q151X alphaB-crystallin is found mainly in the insoluble fraction of cell extracts from transient transfected cells, due to the formation of cytoplasmic aggregates.

Protein glutathionylation and oxidative stress

Liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) demonstrated that glutathionyl hemoglobin (Hb) levels are increased in patients with diabetes, hyperlipidemia, uremia and Friedreich's ataxia. Glutathionylation of Hb is enhanced by oxidative stress. High performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) have also been developed for the quantification of glutathionyl Hb. Glutathionyl-lens proteins were detected in uremic patients and cataractous aged subjects. Glutathionylation of numerous enzymes is induced by oxidative stress, reduces their catalytic activities and may be involved in protection from the damaging effects of oxidative agents. Thioredoxin, glutaredoxin (thioltransferase) and protein disulfide isomerase are the key enzymes in controlling cellular oxidative stress that catalyze reduction of glutathionyl protein disulfide bonds. Thus, protein glutathionylation is closely associated with oxidative stress.

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.

The identification of a reaction site of glutathione mixed-disulphide formation on gammaS-crystallin in human lens

The glutathionylation of human lens proteins was examined by Western-blot analysis with an anti-GSH antibody and scanning. Several different glutathionylated proteins were observed, and a 47 kDa band was of particular interest. This band did not appear after SDS/PAGE under reducing conditions, suggesting that it was a glutathionylated fraction. The 47 kDa band was found principally in the outer part of the lens, the cortex, but not in the lens nucleus where older proteins are present. The 47 kDa component was composed of betaB1-, betaB2- and gammaS-crystallin, with the gammaS-crystallin having glutathione bound at Cys-82 and at Cys-22, Cys-24 or Cys-26. We conclude that when glutathione becomes bound to gammaS-crystallin, it causes it to bind in turn to the beta-crystallin polypeptides to form a dimer.

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.

In vivo carbamylation and acetylation of water-soluble human lens alphaB-crystallin lysine 92

Several post-translational modifications of lysine residues of lens proteins have been implicated in cataractogenesis. In the present study, the molecular weight of an alpha-crystallin isolated from the water-soluble portion of a cataractous human eye lens indicated that it was a modified alphaB-crystallin. Further analysis by mass spectrometry of tryptic digests of this modified protein showed that Lys 92 was modified and that the sample was structurally heterogeneous. Lys 92 was acetylated in one population and carbamylated in another. Although carbamylation of lens crystallins has been predicted, this is the first documentation of in vivo carbamylation of a specific site. These results are also the first documentation of in vivo lysine acetylation of alphaB-crystallin. Both modifications alter the net charge on alphaB-crystallin, a feature that may have significance to cataractogenesis.

Resistance of human betaB2-crystallin to in vivo modification

Post-translational modifications and/or structural changes induced by modifications are likely causes of the decrease in crystallin solubility associated with aging and the development of cataract. Characterization of human lens crystallins by mass spectrometry has demonstrated that betaB2-crystallin undergoes less modification than any of the other crystallins. As the lens ages, betaB2-crystallin retains its hydrophilic N-terminus while the hydrophilic C-termini of alpha-crystallins and large portions of the N-termini of betaA3/A1 and betaB1 are truncated. The hydrophilic terminal regions of crystallins contribute to their solubility. Furthermore, deamidation and disulfide bond formation, other modifications that may affect solubility by altering conformation, are less extensive in betaB2 than in the other crystallins. This resistance to modification results in higher levels of betaB2 compared with the other crystallins in the water-soluble fraction of older lenses. The solubility of betaB2 and its propensity to form non-covalent associations with less soluble beta-crystallins may contribute to the solubility of the other beta-crystallins. A current hypothesis is that the chaperone-like properties of alpha-crystallins contribute to lens crystallin solubility, particularly in younger lenses. In older lenses, where most of the alpha-crystallins have become water-insoluble, betaB2-crystallins may play a dominant role in lens crystallin solubility.

The major in vivo modifications of the human water-insoluble lens crystallins are disulfide bonds, deamidation, methionine oxidation and backbone cleavage

This investigation of the water-insoluble crystallins from human lenses has used multiple chromatographic separations to obtain proteins of sufficient purity for mass spectrometric analysis. Each fraction was analysed to determine the molecular masses of the constituent proteins as well as peptides in tryptic digests of these proteins. The major components of the water-insoluble crystallins were identified as alphaA- and alphaB-crystallins. In addition, gammaS-, betaB1-, gammaD-, betaA3/A1- and betaB2-crystallins were found, in order of decreasing abundance. Although there was evidence of some backbone cleavage, the predominant forms of alphaA-, alphaB, betaB2-, gammaS- and gammaD-crystallins were the intact polypeptide chains. The major modifications distinguishing the water-soluble crystallins were increased disulfide bonding, oxidation of Met, deamidation of Gln and Asn and backbone cleavage. Of the many reactions hypothesized to lead to crystallin insolubility and cataract, these results most strongly support metal-catalysed oxidation, deamidation and truncation as initiators of conformational changes that favor aggregation.

Tracking pathology with proteomics: identification of in vivo degradation products of alphaB-crystallin

Soemmerring's ring is one form of "after cataract" that can occur following cataract surgery. The ring structure is formed by adherence of the anterior lens capsule to the posterior lens capsule. Epithelial cells remaining after surgery differentiate into lens fiber cells but the resulting tissue mass does not remain transparent. The protein in normal lens and in Soemmerring's rings from four individuals was analyzed using two-dimensional (2-D) gel electrophoresis, matrix assisted laser desorption/ionization-time of-flight-mass spectrometry (MALDI-TOF-MS) and image analysis with Phoretix software. The 2-D protein patterns of the Soemmerring's rings were generally similar to that of cortical fiber cells of normal human lens with some notable exceptions. Several post-translationally modified forms of alphaB-crystallin(1-175) were identified. Two degradation products, alphaB-crystallin(1-170) and alphaB-crystallin(1-174), each make up 9.5-27% of the total alphaB-crystallin in the Soemmerring's rings and less than 1% in the normal lenses. Other modified forms of alphaB-crystallin are aberrant in the fiber cells of the Soemmerring rings relative to normal lens.

Human lens beta-crystallin solubility

The human lens is composed primarily of water and proteins called crystallins. Insolubility of these crystallins is correlated with aging and cataractogenesis. The alpha-crystallins have chaperone-like activity in maintaining the solubility of denatured beta- and gamma-crystallins. One established test of this chaperone activity is the ability of alpha-crystallin to prevent thermal destabilization of beta-crystallins. Several studies have addressed the effects of structural modifications of alpha-crystallin on chaperone activity, but little is known about the solubilities of the various beta-crystallins or the effects of post-translational modifications. Understanding the solubilities of different forms of beta-crystallins is important to elucidating the mechanism of chaperone activity. In this study, the solubilities of beta-crystallins were examined. The beta-crystallins included the gene products of betaB2, betaA1/A3, betaA4, and betaB1 as well as forms modified in vivo. Analysis of the beta-crystallins by high performance liquid chromatography and mass spectrometry before and after heating revealed large differences in the relative solubilities of the beta-crystallins. These results demonstrate a decreased solubility of specific beta-crystallins and post-translational modifications that may play a role in the crystallin insolubility associated with aging and cataract.

An atypical form of alphaB-crystallin is present in high concentration in some human cataractous lenses. Identification and characterization of aberrant N- and C-terminal processing

Two unique polypeptides, 22.4 and 16.4 kDa, were prominent in some human cataracts. Both proteins were identified as modified forms of the small heat shock protein, alphaB-crystallin. The concentration of total alphaB-crystallin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alphaB(g). Mass spectrometric analyses of tryptic and Asp-N digests showed alphaB(g) is alphaB-crystallin minus the C-terminal lysine. alphaB(g) constituted 10-90% of the total alphaB-crystallin in these cataracts and was preferentially phosphorylated over the typical form of alphaB-crystallin. Human alphaB(g) and alphaB-crystallin were cloned and expressed in Escherichia coli. The differences in electrophoretic mobility and the large difference in native pI values suggest some structural differences exist. The chaperone-like activity of recombinant human alphaB(g) was comparable to that of recombinant human alphaB-crystallin in preventing the aggregation of lactalbumin induced by dithiothreitol. The mechanism involved in generating alphaB(g) is not known, but a premature termination of the alphaB-crystallin gene was ruled out by sequencing the polymerase chain reaction products of the last exon for the alphaB-crystallin gene from lenses containing alphaB(g). The 16.4-kDa protein was an N-terminally truncated fragment of alphaB(g). The high concentration of alphaB-crystallin in these cataracts is the first observation of this kind in human lenses.

Intrapolypeptide disulfides in human alphaA-crystallin and their effect on chaperone-like function

We report studies on the role of protein-protein disulfides (PSSP) in the age-related loss of chaperone activity of alpha-crystallins. AlphaL-crystallin fraction was isolated from human lenses of different ages and the chaperone-like activity was determined before and after treatment with glutathione reductase (GR) and NADPH. The results confirmed an age-dependent decrease in chaperone-like function and significant improvement of this function by GR treatment. Electrospray ionization mass spectrometric (ESIMS) analysis of alphaA-crystallin suggested the presence of very little protein-glutathione mixed disulfides. ESIMS analysis of Asp-N digests of alphaA-crystallin revealed that nearly all the remaining portion of Cys-131 and Cys-142 of alphaA-crystallin was present in the form of intrapolypeptide disulfide bonds. These results show for the first time that predominantly disulfide bonds formed during aging contribute to the age-dependent loss in chaperone activity of alpha-crystallin in human lenses.

Determination of glutathionyl hemoglobin in hemodialysis patients using electrospray ionization liquid chromatography-mass spectrometry

We first detected glutathionyl hemoglobin (Hb) beta-chain in hemodialysis patients and healthy subjects using electrospray ionization liquid chromatography-mass spectrometry. The ratio of glutathionyl Hb beta-chain to total beta-chain was markedly increased in the hemodialysis patients as compared with healthy subjects. Glutathionyl Hb will be used as a new clinical marker of oxidative stress.

Correlation of nuclear color and opalescence with protein S-thiolation in human lenses

Human lens nuclei were collected during routine cataract surgery and used to study the role of oxidation in cataract formation and brunescence. This study focused on the comparison of the intensities of nuclear opacity and pigmentation (brunescence) with the changes in free glutathione (GSH) and the three species of protein-thiol mixed disulfides: protein-S-S-glutathione (PSSG), protein-S S-cysteine (PSSC) and protein-S-S-gamma-glutamylcysteine (PSSGC). Eighty-one freshly excised human lens nuclei from a population with a mean age of 77 were used. The nuclear color was graded using the CCRG system, ranging from yellow to dark brown. The nuclear cataract opalescence of these lenses was also graded using the LOCS II system, ranging from LOCS II NO-1 to NO-4. Three normal human lenses (average age of 88 yr) were also included in the study as controls. The nuclear samples were each analyzed for free GSH and protein-thiol mixed disulfides, respectively. It was found that nuclear GSH decreased as the nuclear color increased from yellow to dark brown (from 0.73+/-0.13 to 0.13+/-0.03 micromole g wet wt-1) and as the nuclear opalescence increased from NO.1 to NO.4 (from 0. 80+/-0.19 to 0.20+/-0.01 micromole g wet wt-1). All these values were lower than that of GSH in normal controls (1.43+/-0.59 micromole g wet wt-1). Levels of both PSSG and PSSC progressively increased, however, as the nuclear color intensified. PSSG increased from 0.29+/-0.05 to 0.91+/-0.11 micromole g wet wt-1while PSSC increased from 0.13+/-0.04 to 0.41+/- 0.06 micromole g wet wt-1. PSSGC concentration progressively increased with increases in both nuclear pigmentation (from 0.05+/-0.01 to 0.23+/-0.05 micromole g wet wt-1) and nuclear opacity (from 0.02+/-0.00 to 0.20+/-0.02 micromole g wet wt-1). In comparison, normal controls had lower levels of all three mixed disulfide species: PSSG, 0.22+/-0.06; PSSC, 0.08+/-0.02; PSSGC, 0.02+/-0.06 micromole g wet wt-1, respectively. The correlation of lens nuclear color and opalescence intensity with nuclear protein S-thiolation indicates that protein-thiol mixed disulfides may play an important role in cataractogenesis and development of brunescence in human lenses.

Thiolation of the gammaB-crystallins in intact bovine lens exposed to hydrogen peroxide

Oxidative damage of the lens causes disulfide bonds between cysteinyl residues of lens proteins and thiols such as glutathione and cysteine, which may lead to cataract. The effect of H2O2 oxidation was determined by comparing bovine lenses incubated with and without 30 mM H2O2. The H2O2 treatment decreased the glutathione and increased the protein-glutathione and protein-cysteine disulfides in the lens. The molecular mass of the gammaB-crystallin isolated from lenses, not treated with H2O2, agreed with the published sequence (Mr 20,966). Some lenses also had a less abundant gammaB-crystallin component 305 Da higher (Mr 21,270), suggesting the presence of a glutathione adduct. The gammaB-crystallins from H2O2 treated lenses had three components, the major one with one GSH adduct, another one with the mass of unmodified gammaB-crystallin, and a third with a mass consistent with addition of two GSH adducts. Mass spectrometric analysis of tryptic peptides of gammaB-crystallins from different lenses indicated that the +305 Da modifications were not at a specific cysteine. For the lenses incubated without H2O2, there was evidence of adducts at Cys-41 and in peptide 10-31, which includes 3 cysteines. Analysis of modified peptide 10-31 by tandem mass spectrometry showed GSH adducts at Cys-15, Cys-18, and Cys-22. In addition, gammaB-crystallins from H2O2-treated lenses had an adduct at Cys-109, partial oxidation at all 7 Met residues, and evidence for two disulfide bonds.

Deamidation and disulfide bonding in human lens gamma-crystallins

Detailed analysis of the three gamma-crystallins present in the water-soluble portion of human lenses, gammaS, gammaD and gammaC, has identified disulfide bonding and deamidation as the major post-translational modifications of these crystallins. Chromatographic and mass spectrometric techniques were used to isolate and identify water-soluble gamma-crystallins from normal lenses, ages 32 week gestation, 0 day old, 4 day old, 19, 31, 45 and 55 year old. The amino acid sequences of the gamma-crystallins were confirmed and/or corrected by mass spectrometric analysis of peptides produced by enzymatic digestion or chemical fragmentation of the isolated crystallins. The molecular weight of peptides were also used to identify, locate and quantify modifications. Each of the gamma-crystallins had two disulfide bonds as well as several deamidated glutamine and asparagine residues. The extent of disulfide bond formation and deamidation appeared to increase with the age of the lens. This examination of normal human lens gamma-crystallins, the first detailed characterization of the gamma-crystallins, will provide a basis for comparison with modifications found in the water-insoluble portion and in cataractous lenses.

In vivo modification of the C-terminal lysine of human lens alphaB-crystallin

Both the structural and chaperone-like properties of lens alpha-crystallins have been implicated in maintaining lens transparency. Modifications of lens alpha-crystallins may lead to formation of cataract by affecting the close-packing of the crystallins or by reducing the chaperone-like activity of the alpha-crystallins. A previously unreported modified alphaB-crystallin, whose molecular weight is 72 u greater than unmodified alphaB-crystallin, has been isolated from human lenses by size exclusion chromatography, reversed phase HPLC and ion exchange HPLC. Approximately one nanomole of this modified alphaB-crystallin was obtained from each of five human eye lenses. Molecular weight determinations of peptides produced by digestion with trypsin or endoproteinase Asp-N showed that the modification is in the C-terminal region of alphaB-crystallin. The fragmentation pattern of peptides from the C-terminal region, analysed by tandem mass spectrometry, located the modification of the epsilon-amino group of the C-terminal lysine. The elemental composition of this modification, determined from its exact mass, is C3H4O2. Because this modification decreases the net charge of alphaB-crystallin by one unit, and because the C-terminus has been implicated in the chaperone activity attributed to alphaB-crystallin, this modification at Lys 175 may have a significant role in cataractogenesis.

Mass spectrometry in the search for uremic toxins

This article reviews the literature on the mass spectrometry (MS) that has been used in the research of uremic toxins. Gas chromatography/mass spectrometry (GC/MS) has been most often used for the analysis of low-molecular-weight compounds in uremic blood such as organic acids, phenols, and polyols. However, it cannot be used for the analysis of middle- to high-molecular-weight substances or for involatile compounds. The development of fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) has made possible the analysis of middle-molecules and involatile low-molecular-weight substances such as peptides and nucleosides. The development of atmospheric pressure chemical ionization (APCI) has also lead to the analysis of involatile low-molecular-weight substances. The recent advances in ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI), have permitted the MS analysis of high-molecular-weight substances such as beta 2-microglobulin, a major component of dialysis amyloid. Liquid chromatography/mass spectrometry (LC/MS), using ESI, APCI, or FAB as an ionization method, is currently the preferred method for the analysis of low- to high-molecular-weight substances in uremic blood. ESI-LC/MS and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOFMS) are useful for elucidating the structure of post-translationally modified proteins obtained from the blood and tissues of uremic patients. Post-translational modification such as the formation of advanced glycation end-products and carbamoylation is enhanced in uremic patients, and is considered to be responsible for some uremic symptoms. Laser microprobe MS is unique in its capability for the two-dimensional detection of atoms such as aluminum in a tissue section obtained from uremic patients. This review focuses on the mainstream research for discovering uremic toxins, specific uremic toxins identified or quantified using MS, and the MS analysis of post-translationally modified proteins in uremia. These studies have provided ample evidence that MS has played an important role in the search for uremic toxins.

The sequence of human betaB1-crystallin cDNA allows mass spectrometric detection of betaB1 protein missing portions of its N-terminal extension

The sequence of human betaB1-crystallin cDNA encoded a protein of 251 amino acids in length. Mass spectrometric analysis of intact betaB1 from young human lens confirmed the deduced amino acid sequence. Lenses of human donors newborn to 27 years of age also contained partially degraded forms of betaB1 missing 15, 33, 34, 35, 36, 39, 40, and 41 amino acid residues from their N-terminal extensions. The similarity of the cleavage site between residues 15 and 16 in human betaB1 to the cleavage occurring in bovine betaB1 suggested that lenses of both species may contain a similar proteolytic activity. The remaining cleavage sites occurring in human betaB1 did not closely match those occurring in other species, possibly due to the widely divergent amino acid sequence of the N-terminal extension of betaB1 amoung species. Results from animal models suggest that cleavage of the N-terminal extension of betaB1-crystallin could enhance protein insolubilization and cataract in lens. However, the presence of partially degraded betaB1-crystallins in both water-soluble and water-insoluble fractions of lenses of young donors suggested that the rate that proteolyzed betaB1-crystallins become water-insoluble is relatively slow in humans.