Age-dependent changes in the heat-stable crystallin, beta Bp, of the human lens

Associations of seroreactivity against crystallin proteins with disease activity and cataract in patients with uveitis

PURPOSE

betaB1-crystallin is a putative target of an autoantibody observed in a subset of patients with uveitis. The purpose of this study was to determine whether seroreactivity against betaB1 or other specific purified crystallin proteins is observed in patients with uveitis and whether this reactivity is associated with either cataract or active intraocular inflammation.

METHODS

Sera from patients with uveitis were tested for IgG antibodies with reactivity against alphaA-, alphaB-, betaB1-, or betaB2-crystallin proteins using a modified slot-blot protocol. Ophthalmic evaluations included analysis of the degree of intraocular inflammation and assessment of lens opacity by the Lens Opacities Classification System (LOCS) III. Positive anti-crystallin reactivity was defined as greater than the mean + 2 SD of the reactivity of a commercially available control serum panel. Statistical analysis was performed with the Fisher exact test, Kruskal-Wallis test, and Student's t-test.

RESULTS

IgG antibodies against alphaA-, alphaB-, or betaB1-crystallin were identified in 70% of 39 subjects; in contrast, only 30% of the control sera exhibited reactivity against one or more of these crystallin proteins (P <or= 0.01). Seroreactivity against alphaA-, alphaB-, or betaB1-, but not betaB2-crystallin was related to active anterior segment inflammation. Seroreactivity against alphaB and betaB1 was significantly related to cortical cataract (P <or= 0.05).

CONCLUSIONS

Serum antibodies against specific crystallin proteins are present in most patients with uveitis. The relationship between the presence of specific anti-crystallin antibodies and active inflammation may indicate a role for these autoantibodies in uveitis pathogenesis.

BetaB2-crystallin undergoes extensive truncation during aging in human lenses

Based on the present literature, it is unclear whether betaB2-crystallin undergoes age-related truncation in human lenses. To answer this question, the purpose of this study was to determine in vivo truncation of betaB2-crystallin in human lenses during aging by examining its fragments in the beta(H)-crystallin fraction. The WS-protein fraction was isolated from lenses of desired ages and separated by a size-exclusion Agarose A 1.5m column to recover alpha-, beta(H)-, beta(L)-, and gamma-crystallin fractions. The beta(H)-crystallin fractions, isolated from lenses of 24- and 70-year-old donors, were utilized for two-dimensional (2D) gel electrophoresis (isoelectric focusing in the first dimension followed by SDS-PAGE in the second dimension). The partial N-terminal sequences of the desired fragments (Molecular weights [M(r)]<18-19kDa) from a 2D-gel of WS-proteins from lenses of a 70-year-old donor were determined. More than 37 crystallin fragments with M(r) between 4 and 19kDa were observed on a 2D-gel. Nine fragments in beta(H)-crystallin fraction were from betaB2-crystallin but additional single fragments of alphaA-, gammas-, betaA4, and of either gammaB-, gammaC- or gammaD-crystallins were also observed. Seven cleavage sites in the betaB2-crystallin were identified, which included two sites at Q(7)-A(8) and A(8)-G(9) bonds in the N-terminal extension, two sites at E(46)-K(47) and G(49)-S(50) bonds in the motif 1, one site at S(94) -S(95) in the motif 2, and two sites at N(115)-F(116) and Q(135)-Y(136) in motif 3. No fragments with cleavage in the motif 4 and C-terminal extension of betaB2-crystallin were seen. Apparently, three betaB2-crystallin fragments with only N-terminal cleavage and five with both N- and C-terminal cleavages were observed. Additional fragments with cleavage sites at Q(54)-Y(55) in alphaA-crystallin, at E(112)-N(113) in betaA4-crystallin, at G(4)-T(5) in gammas-crystallin, at M(69)-G(70) in either gammaB-, gammaC- or gammaD-crystallins (three have identical sequences at the cleaved bond), and at G(1)-K(2) in gammaB or gammaC (both have identical sequences at the cleavage site) were observed.Conclusions. The results showed that betaB2-crystallin undergoes age-related truncation producing fragments with M(r) between 4 and 19kDa that existed in the beta(H)-crystallin oligomer. The beta(H)-crystallin fraction also contained single fragments of alpha-, betaA4-, gammas-, and other gamma-crystallins.

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.

Studies on trypsin-modified bovine and human lens acylpeptide hydrolase

Acylpeptide hydrolase removes the N -acetylated amino acids from the peptide substrates but not from intact proteins. Cleavage between amino acid residues 203--204 of the native acylpeptide hydrolase results in the formation of a 55 kDa truncated active enzyme in the bovine lens, in vivo. In this study we explored the hydrolytic properties of the truncated enzyme using lens beta- and gamma-crystallins as substrates. SDS--PAGE analysis indicated that the beta B2-crystallin was cleaved by truncated acylpeptide hydrolase into several protein fragments (10--26 kDa). No cleavage of the gamma-crystallins was observed under similar conditions. Both the acylpeptide hydrolase activity and the protease activity of the 55 kDa enzyme were completely inhibited by diisopropylfluorophosphate, p -chloromercuribenzoate and ebelactone, and moderately inhibited by N -tosyl phenylalanine chloromethyl ketone. SDS--PAGE analysis followed by fluorography of ((3)H) diisopropylfluorophosphate labeled human lens acylpeptide hydrolase preparation showed the presence of the 55 kDa truncated form of the enzyme, as observed in the bovine lens. The peptide (d)-AIKGDQFL-NH(2)--the amino acid sequence 200--207 of the native bovine acylpeptide hydrolase with an in vivo cleavage site of native protein--was hydrolysed by the lens protease(s) suggesting that the in vivo generation of the 55 kDa acylpeptide hydrolase may be mediated through a proteolytic processing. The protease(s) responsible for the cleavage of this peptide was inhibited by diisopropylfluorophosphate and p -chloromercuribenzoate.

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.

Age-related degradation of betaA3/A1-crystallin in human lenses

The aim of this study was to determine age-related degradation of betaA3/A1-crystallin in human lenses. The betaA3/A1-crystallin fragments were identified by Western blot analysis using two site-specific anti-betaA3/A1-crystallin antibodies. The first antibody was raised against a N-terminal region (residues 35-66), and the second to the C-terminal (residues 203-214) region of the crystallin. During the analyses, either preparative SDS-PAGE-separated fragments from betaH-crystallin fraction or water-soluble (WS) protein fractions from lenses of different aged donors were used. In lenses from 27- to 30-year-old donors, four major crystallin fragments of about 5, 16, 17, and 18 kDa immunoreacted with the anti-betaA3/A1-N-terminal antibody, suggesting their intact N-terminus but cleaved C-terminus. A similar analysis with the anti-betaA3/A1-C-terminal antibody identified 15-, 18-, 19-, and 20-kDa species and also five species between 4 and 11 kDa that had intact C-terminus but cleaved N-terminus. In lenses from a 5-year-old donor only two crystallin species, a major 15-kDa and a minor 18-kDa species, showed an intact N-terminus and cleaved C-terminus, whereas, eight species with Mr's between 4 and 19 kDa exhibited intact C-terminus but cleaved N-terminus. Upon two-dimensional gel electrophoresis of a betaH-crystallin fraction from the lenses of a 70-year-old donor, a degradation profile almost similar to the crystallin mentioned above was observed. However, the existence of multiple spots with identical Mr's of truncated betaA3/A1-crystallin species on the 2D-gel suggests their existence as isoforms (identical size species with different charges) because of post-translational modifications. Five species of 4, 6, 11, 15, and 18 kDa showed an identical partial N-terminal sequence of N-F-Q-G, suggesting cleavage at the E39-N40 bond during their production. Together, the data suggest that the majority of age-related cleavages in betaA3/A1-crystallin occur at the N-terminal region, with a major cleavage site at the E39-N40 bond generating some of these fragments.

Age-related changes in human lens crystallins identified by HPLC and mass spectrometry

Analysis of water-soluble crystallins from human lenses, ages 32 week fetal to 55 years has led to identification of the major modifications of the proteins comprising the lens. These modifications were identified by the masses of the proteins determined by electrospray ionization mass spectrometry after the proteins were separated by gel filtration and reversed phase high performance liquid chromatography. Examination of all the proteins isolated from the water soluble portion demonstrated that the major age-related modifications causing significant alteration in the molecular weights of the lens crystallins include truncation of the N-termini of beta B1, beta A3 and beta A1, and partial phosphorylation and C-terminal degradation of alpha-crystallins. N-terminal degradation of beta B1, beta A3 and beta A1 was evident in human lenses less than one year old, and the proportion of these truncated proteins became greater with age. Phosphorylation of alpha A- and alpha B-crystallins increased from the fetal to the 3 year old lens, but did not change with further aging. Minor components indicating truncation of the C-termini of alpha-crystallins were found in older lenses. In contrast to beta B1, beta A3 and beta A1, the masses of the major species of alpha A, alpha B, beta B2, beta A4, gamma S, gamma C, and gamma D did not change with aging. This suggested that the major modifications to these crystallins are limited to deamidation and possibly intra-molecular disulfide bonds. These data, in conjunction with the data in the accompanying manuscript, established deamidation as a common modification, since deamidation, which causes only a one dalton change in mass, is the only modification that is consistent with the absence of a detectable change in molecular weight and the observed increased acidity demonstrated in the two-dimensional gels of the accompanying paper. Other age related changes included a decrease in beta B3 (M(r) 24224), a major component of the fetal lens, which was not detected in lenses older than 3 years, and increases in the ratios of alpha B:alpha A and gamma S:gamma C.

Size of human lens beta-crystallin aggregates are distinguished by N-terminal truncation of betaB1

The aggregates formed by the interactions of the human lens beta-crystallins have been particularly difficult to characterize because the beta-crystallins comprise several proteins of similar structure and molecular weight and because their sequences were not known until recently. Previously, it could not be ascertained whether the species of various acidities were different proteins or modifications of the same proteins. The recent determination of the sequences permits calculation of molecular weights and unambiguous identification of the various beta-crystallins and their modified forms by mass spectrometry. In this investigation, the components of the three sizes of beta-crystallin aggregates, beta1 (approximately 150,000), beta2 (approximately 92,000), and beta3 (approximately 46,000), were determined. The principal differences among the different beta-crystallin aggregates was the presence of betaA4 in beta1 and beta2, but not beta3, and the length of the N-terminal extension of betaB1. The size of the beta-crystallin aggregate correlated with the length of the N-terminal extension of betaB1, indicating that the flexible N terminus of betaB1 is critical to the formation of higher molecular weight aggregates of beta-crystallins. Separation of the components by ion exchange under non-denaturing conditions showed that betaB2 occurs as homo-dimers and homo-tetramers as well as contributing to hetero-oligomers. Other beta-crystallins were present only as hetero-oligomers.

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.

The amine-donor substrate specificity of tissue-type transglutaminase. Influence of amino acid residues flanking the amine-donor lysine residue

The amine-donor substrate specificity of tissue-type transglutaminase has been studied in a series of recombinant alpha A-crystallin mutants. These mutant proteins have been provided with a potential substrate lysine residue, flanked by different amino acid residues, in the C-terminal extended arm of alpha A-crystallin. A biotinylated amine-acceptor hexapeptide was used as a probe for labelling the amine-donor sites. Wild-type bovine alpha A-crystallin does not function as an amine-donor substrate for tissue-type transglutaminase. Yet, upon introduction of a lysine residue at the C-terminal or penultimate position, all mutant alpha A-crystallins act as amine-donor substrates, although to different extents. This shows that accessibility is the primary requirement for a lysine residue to function as an amine-donor substrate for transglutaminase and that the enzyme has a broad tolerance towards the neighbouring residues. However, the nature of the flanking amino acid residues does clearly affect the reactivity of the substrate lysine residue. Notably, we found that a proline or glycine residue in front of the substrate lysine has a strong adverse effect on the substrate reactivity as compared to a preceding leucine, serine, alanine or arginine residue.

A biochemical characterization of the photophore lenses of the midshipman fish, Porichthys notatus Girard

The present study is a biochemical characterization of the photophore lenses of the midshipman fish, Porichthys notatus, a species that bears 800 photophores distributed over the body surface. The biochemical properties of the photophore lenses were compared with those of the eye lens with which they share a similar developmental origin and analogous function. To achieve a high refractive index, the vertebrate eye lens has a relatively high concentration of structural proteins (20-50%, depending on species) and a simple protein composition, that is, relatively few proteins are synthesized in comparison to other tissues. Similarly, the photophore lenses of P. notatus had a relatively high protein concentration (average = 29%, n = 5) and approximately 60% of the total soluble protein was represented by two subunit species of 33 kD and 35 kD on denaturing polyacrylamide gels. The structural proteins of the eye lens are of two principle types: 1) beta and gamma polypeptides which belong to vertebrate lens-specific crystallin families, and, 2) enzymes recruited into the lens which take on the function of structural proteins. Here, we report that the two major photophore lens subunits of 33 kD and 35 kD are biochemically similar to each other, but are clearly distinct from any of the previously characterized crystallins. Therefore, we propose that photophore lenses appear to recruit a novel protein.

Lens fiber cell differentiation and expression of crystallins in co-cultures of human fetal lens epithelial cells and fibroblasts

Growth of the ocular lens is directed by the division and differentiation of a single layer of epithelial cells located at the equatorial region. It is conceivable that this region of the lens capsule presents a special microenvironment modulated by molecular cues emanating from the surrounding tissues. In an effort to investigate the source and nature of these molecular cues, we co-cultured human fetal lens epithelial cells and fibroblasts derived from the ciliary body. We observed morphological differentiation as evidenced by the appearance of differentiating lentoid structures associated with fibroblasts. Characterization of the expression of lens-specific proteins revealed that in addition to alpha B-crystallin, these lentoid structures contain the lens fiber cell-specific proteins, alpha A-crystallin, beta B2-crystallin and gamma S-crystallin. None of these crystallins could be found in the surrounding undifferentiated lens epithelial cells. Interestingly, alpha B-crystallin usually present in lens epithelial cells when cultured alone, was found to be markedly decreased, both in synthesis and content in the cells surrounding the differentiated structures, suggesting that the process of differentiation in vitro may concomitantly produce a factor(s) which modulates alpha B-crystallin expression in these cells.

Induction of de novo synthesis of crystalline lenses in aphakic rabbits

The mammalian lens, like other ectodermal tissues, can regenerate itself given the proper environment. Endocapsular phacoemulsification of adult rabbit lenses was performed. A lens capsular bag with posterior and anterior lens capsule relatively intact was left in the eye. Regrowth of material in the capsular bag was followed by slit lamp biomicroscopy and photography over a 12-month period. Histopathology of the new material showed regions of relatively normal epithelial cells and lens fibers as well as regions where growth was irregular. All major lens crystallin classes were present in the regenerated lens. Several specific crystallin subunits, known to arise by post-translational modification of primary gene products, were absent or present in abnormally low concentrations.

The effects of near-UV radiation on human lens beta-crystallins: protein structural changes and the production of O2- and H2O2

beta-Crystallins (beta 1-, beta 2- and beta 3-crystallin) comprise nearly half the protein of the human lens. The effect of near-UV radiation, which is one of the possible risk factors in cataract formation, on the beta-crystallins is investigated in this study. Protein intersubunit crosslinking, change in charge of the protein subunits to more acidic species and changes in protein tertiary structure (conformation) by 300 nm irradiation are reported. The fluorescence yield of protein tryptophan residues decreases by 300 nm irradiation. There is an increase in nontryptophan fluorescence (lambda cx 340 nm, lambda cm 400-600 nm), and in protein absorption at 340 nm, due to the formation of tryptophan photooxidation products. Both tryptophan and its oxidation products can be photoexcited by 300 nm irradiation and the latter are known to be good photosensitizers. The results provide evidence for the generation of H2O2 in the irradiated human beta-crystallin solutions by the Type I photosensitizing action of the chromophores absorbing at 300 nm. The H2O2 is generated via the intermediate production of O2 anion; the latter spontaneously dismutates to H2O2, presumably via O2- protein interactions. The amount of H2O2 generated per absorbed photon is compared for various solutions of beta 1-, beta 2- and beta 3-crystallins from human lenses of different age.

Human fetal lens epithelial cells in culture: an in vitro model for the study of crystallin expression and lens differentiation

We have cultured and passaged human fetal lens epithelial cells. Cultured cells exhibited hexagonal, cuboidal shape typical of epithelial cells. Unlike previous observations made with cultured mammalian lens epithelial cells, indirect immunofluorescence and temporal analysis of 35S-labeled proteins demonstrated undiminished levels of alpha B-crystallin in primary, secondary, and tertiary cultures. Among the alpha-crystallins only alpha B synthesis was detected. Two dimensional gel electrophoresis indicated the presence of alpha B2 and no alpha B1. beta B2-crystallin, a fiber cell specific protein hardly detectable in primary cultures, increased significantly upon passaging. Human fetal lens epithelial cell cultures, described in this report, thus present a useful in vitro model for the study of lens epithelial cell differentiation and its pathological manifestations.

Human lens membrane proteinase: purification and age-related distributional changes in the water-soluble and insoluble protein fractions

A proteinase from human lens membrane was purified by a procedure previously developed for a similar proteinase from bovine lenses (Srivastava, 1988a). The purification of the human proteinase was achieved by solubilization of the enzyme from membranes with 2% sodium deoxycholate followed by two consecutive passages through an Agarose A-1.5 m column. The purified proteinase exhibited molecular weight of 38 kDa on a SDS-polyacrylamide gel. A polyclonal antiserum was raised against bovine lens membrane proteinase, and used as a probe to examine distribution of the enzyme among water-soluble and insoluble proteins of human lenses of different ages. The antiserum had strict specificity to the human membrane proteinase as it showed immunoreaction to only the proteinase among human membrane proteins and crystallins. In addition, the antiserum also inhibited the proteinase activity on incubation. The Western blot of water-soluble proteins from 2-yr-old lens showed a 22 kDa immunoreactive protein, but an additional protein of 43 kD in lenses older than 19 years was observed. A similar Western blot analysis of the water-insoluble proteins from these lenses showed a single protein of 18 kDa that was identified as the subunit of the bovine lens membrane proteinase (Srivastava, 1988a). Furthermore, the immunoreactive 18 kD protein of the water-insoluble protein fractions could be solubilized with urea. The proteinase activity was found to increase with aging, as judged by the extraction of the enzyme with 2% deoxycholate from membranes of lenses of different ages and proteinase activity determination. Similarly, an age-related increase in the immunoreaction was also observed on measuring radio-iodinated protein A bound to an immunoreactive 18 kD protein of the water-insoluble protein fractions.

Maintenance of the synthesis of alpha B-crystallin and progressive expression of beta Bp-crystallin in human fetal lens epithelial cells in culture

We have cultured and maintained human fetal lens epithelial cells for several months in primary, secondary, and tertiary culture(s). These cells show unabated synthesis of alpha B-crystallin (alpha B), a lens epithelial cell-specific marker, and progressive expression of beta Bp-crystallin (beta Bp), a major polypeptide of the differentiated lens fiber cells in vivo. Interestingly, the expression of beta Bp was found to be dependent on subculturing of the cells and not on the age of cultures. These observations demonstrate that human fetal lens epithelial cells can be cultured in vitro without the loss of lens specific characteristics and with commitment to differentiation at the biochemical level.

Age-related increase in concentration and aggregation of degraded polypeptides in human lenses

Proteins from human lenses of various ages were separated into three fractions based on their solubility; water-soluble (WS), water-insoluble-urea-soluble (WI-US) and water-insoluble-urea-insoluble (WI-UI). The SDS-PAGE of these fractions showed increasing quantities of degraded polypeptides (mol. wt; Mr less than 18 kDa) with aging. On separation of degraded polypeptides from the rest of lens crystallins by gel-filtration chromatography under denaturing conditions, the polypeptides showed an age-related concentration increase in the WS and WI-US fractions, which constituted 12-14% and 17-18% of the total proteins respectively. The degraded polypeptides from WI-US fractions of lenses from 50-yr-old donors exhibited five polypeptides ranging in Mr between 3- and 18 kDa. The degraded polypeptides isolated from WS and WI-US fractions of lenses of various ages self-aggregated on storage to protein moieties with Mr ranging between 18- and greater than 1500 kDa. An antiserum was raised against degraded polypeptides isolated from WI-US protein fraction of 50-yr-old human lenses. During the Western blot analyses, the antiserum exhibited specific immuno-reaction with alpha-crystallin and 20-kDa gamma-crystallin species and with degraded polypeptides among various human lens crystallins tested. Similarly, when the self-aggregated degraded polypeptides of WS and WI-US fractions were reacted with the antiserum, several polymeric proteins of increasing Mrs were observed. In addition, when the heavy molecular weight (HMW) proteins from WS proteins and the total WI proteins of lenses of various ages were probed with the antiserum, several polymeric proteins, similar to the in vitro self-aggregated polymers of degraded polypeptides as previously described, were seen. Such polymers were in relatively lower quantities in the total WS proteins of lenses of various ages. These results suggest an apparent age-related polymerization of degraded polypeptides into HMW proteins leading to their insolubilization.

Heat-induced changes in the conformation of alpha- and beta-crystallins: unique thermal stability of alpha-crystallin

Of the crystallin proteins of the lens, the principal subunit of the beta-crystallin, beta B2 (beta Bp), has been considered to be the only heat-stable protein because it does not precipitate upon heating. In our recent investigations, however, we have found that the alpha-crystallin from bovine lenses is not only heat stable but also does not denature at temperatures up to 100 degrees C. Using circular dichroism and fluorescence to monitor the conformational changes of alpha- and beta B2-crystallins upon heating, we found that alpha-crystallin maintains a high degree of structure, whereas the beta B2-crystallin shows a reversible sigmoidal order-disorder transition at about 58 degrees C.

Human beta crystallins: regional and age related changes

The composition of human beta-crystallins displayed specific changes with age and region of the lens. 27 kD and 29 kD human beta-crystallin subunits were singled out for study. The 29 kD beta-crystallin subunit constituted approximately 10% of the total lens crystallins at 8 months of fetal life. Its accumulation decreased steadily to 3.3% during postnatal year 1, to 0.5% by year 5 and to 0.3% thereafter. At all postnatal ages, however, it persisted mainly in the superficial fibers. Thus in a 17-years old lens it made up 1.3% of the superficial fiber soluble protein but was already absent from deep cortical and nuclear fibers. The 27 kD subunit increased steadily from 3.5% at 8 months fetal to 7% at year 5; it then decreased steadily to 1.2% in the 86-year old lens. It persisted in all regions of the lens but decreased markedly in the deep cortical and nuclear fibers with increasing age beginning at 5-17 years of age. Studies on the oligomeric structure of human beta-crystallin must take into account age-related changing quantitative patterns in the subunit polypeptide composition of this lens protein.

The influence of the genotype on the process of ageing of chick lens cells in vitro

We reported previously that changes in crystallin expression in differentiating long-term primary cultures of lens cells from five different chick genotypes are similar to those which occur in vivo between hatching and the 8-week-old adult. These changes followed a similar program in all genotypes but occurred more rapidly in cells from the fast-growing than from the slow-growing genotypes. The present study examines ageing changes in lens cell populations from the same five genotypes, over a 4-6 month period, using long-term serial subcultures. The capacity for lentoid differentiation was progressively lost, but the rate of loss was inversely related to the intrinsic growth rate of the cells of these genotypes, occurring at the first passage in the slowest-growing strain, while fifth passage cells of the fastest-growing strain still retained some lentoid-forming capacity. The rate of loss of crystallin expression was also inversely related to the genetic growth rate, but the sequence of changes appears to be nonrandom, since it was broadly similar in all genotypes, starting with a preferential loss of delta-crystallin, as occurs in vivo; although alpha- and beta-crystallins were undetectable in late dedifferentiated cultures, the capacity of the cells for their synthesis was still present. Cultures from both fast-growing genotypes eventually showed senescence, but those from all three slow-growing genotypes underwent transformation. The major cell component in late cultures of all genotypes was actin.

The binding and inhibition of trypsin by alpha-crystallin

One of the major lens-structural proteins, alpha-crystallin, is a multimeric protein containing 40 subunits of approx. 20 kDa each. There are two subunit types with distinct but similar structures. This protein was capable of inhibiting trypsin, chymotrypsin and elastase, but had no effect on thrombin or kallikrein. Complete inhibition was not observed, but rather plateau levels of inhibition were obtained in each case. Maximum inhibition was observed at a ratio of 1 mol of alpha-crystallin for every 9-10 mol of trypsin. alpha-Crystallin also inhibited the labeling of the active site of trypsin by [3H]diisopropyl fluorophosphate (DFP). Greater than 90% inhibition of DFP labeling was observed at a ratio of 1 mol of alpha-crystallin for every 7-8 mol of trypsin. Both trypsin and [3H]DFP-labeled trypsin formed a complex with alpha-crystallin, as demonstrated by gel-filtration chromatography. The active site of trypsin when bound to alpha-crystallin was still capable of reacting with p-nitrophenyl p-guanidobenzoate and soybean trypsin inhibitor, but was inaccessible to alpha 1-antitrypsin. These data suggest that alpha-crystallin acts as a multivalent modified inhibitor which is consistent with the proposed quaternary structure of alpha-crystallin.

Cleavage from the N-terminal region of beta Bp crystallin during aging of the human lens

Polyclonal antisera have been made to synthetic peptides that correspond to the N-terminal (residues 1-12) and C-terminal (residues 195-204) sequences of bovine beta Bp crystallin. Both anti-beta Bp1-12 and anti-beta Bp195-204 recognize specifically the beta Bp component of bovine lens. In the young human lens, anti-beta Bp195-204 recognizes predominantly the 26,000 MW form of beta Bp, while in older lenses this same antiserum recognizes mainly the 22,000 MW in vivo proteolysis product. In contrast, during aging of the normal human lens anti-beta Bp1-12 recognizes only decreasing amounts of the 26,000 MW form of beta Bp, with no binding to the 22,000 MW form of this polypeptide. These results suggest that during aging of the normal human lens, the N-terminus of beta Bp is the preferred site of in vivo proteolysis.

Beta B1 crystallin is an amine-donor substrate for tissue transglutaminase

The effects of tissue transglutaminase on the water-soluble proteins in bovine lens homogenates are described. Addition of liver transglutaminase and Ca2+ to calf lens homogenates resulted not only in the appearance of 50- and 57-kDa dimers, but also in a decrease in the amount of beta B1 crystallin and the almost complete disappearance of beta B3 and beta A3. This is not the result of Ca2+-induced proteolysis, since histamine completely inhibits this phenomenon. It may be concluded that these polypeptides are involved in beta-crystallin crosslinking by transglutaminase. This notion was confirmed by using beta B1- and beta Bp-specific antisera. Both sera reacted with the 57-kDa dimer; the beta Bp-specific antiserum also reacted with the 50-kDa dimer. No reaction in the region 50-57 kDa was detectable when EDTA was used instead of Ca2+. Using reconstituted mixtures of beta B1- and beta Bp-crystallin chains, and N-terminally truncated derivatives thereof, it was shown that in the beta B1/beta Bp dimer, glutamine residue -9 of beta Bp crosslinks to one of the lysine residues in the N-terminal extension of beta B1.

Covalent changes at the N- and C-terminal regions of gamma crystallin during aging of the normal human lens

Polyclonal antisera have been made to synthetic peptides corresponding to the N- and C-terminal regions of the putative gamma 1-2 gene from human lens. These antisera are specific for gamma crystallin, showing no cross-reactivity with any polypeptides of the alpha- and beta-crystallin fractions. Western blot analysis demonstrates a dramatic decrease in the binding of these antisera to gamma crystallin during aging of the normal human lens, while identical analysis with polyclonal antisera to the major cyanogen bromide fragment (19,000 MW) of human gamma crystallin shows little, if any, change during aging. Together, these demonstrate that antisera to synthetic peptides of the N- and C-terminal regions of the gamma 1-2 gene are very specific probes that can demonstrate extensive covalent modification from both ends of the gamma crystallin molecule during aging of the normal human lens.