Age and cataract-related changes in the heavy molecular weight proteins and gamma crystallin composition of the mouse lens

The effects of hyperbaric oxygen on the crystallins of cultured rabbit lenses: a possible catalytic role for copper

Oxidative effects on lens proteins have been linked with the formation of human age-related cataract, particularly nuclear cataract. This study investigated the effects of hyperbaric oxygen (HBO)-induced oxidative stress on nuclear and cortical alpha-, beta- and gamma-crystallins of cultured rabbit lenses, using high performance liquid chromatography (HPLC). The lenses were treated with 50 atm of either 100% N(2)(control) or 100% O(2)(experimental) for 3, 6, 16 and 48 hr. The levels of reduced glutathione (GSH) and water-soluble (WS) protein decreased more rapidly in the nucleus of the O(2)-treated lens than in the cortex. The first significant loss of WS protein in each of the two regions occurred when levels of GSH had decreased by at least 90% in either the nucleus (at 6 hr) or the cortex (at 16 hr). HPLC analysis of the nuclear WS proteins indicated that beta-crystallins were the first proteins affected by the oxidative stress. Soon after HBO-treatment was initiated (at 6 hr) and prior to insolubilization of protein, nuclear beta- and gamma-crystallins moved to the higher molecular weight alpha-crystallin fraction; 2-D gel electrophoresis and Western blotting indicated the presence of disulfide-crosslinked and non-crosslinked beta- and gamma-crystallins in this fraction. Significantly different HBO-induced effects were observed on lens cortical crystallins compared to those for the nucleus. For example, gamma-crystallins in the cortex shifted very soon after HBO-treatment (at 3 hr) to slightly higher molecular weights, possibly the result of protein/glutathione mixed disulfide formation; however, this phenomenon was not observed in the nucleus. Cortical beta- and gamma-crystallins remained in solution longer than nuclear proteins following HBO-treatment of the lenses, presumably the result of protection from the four-fold higher level of GSH (22 vs 6 m M) present in the lens periphery. Surprisingly, there was no movement of beta- and gamma-crystallins to alpha(H)- and alpha-crystallin fractions in the cortex of the O(2)-treated lens, in contrast to that observed for the nucleus. Cortical crystallins appeared to go directly from being soluble to being insoluble with no high molecular weight intermediate stage. The data suggested a possible chaperone-like function for alpha-crystallin in the nucleus of the stressed lenses, but not in the cortex. HBO-induced effects on lens nuclear supernatants, which mimicked those observed for intact lenses, could be nearly completely prevented by the copper-chelator bathocuproine, but not by the iron-chelator deferoxamine. Overall, the results provide additional evidence demonstrating an increased susceptibility of the lens nucleus to oxidative stress; the greater protective ability of the cortex may be linked to a higher capacity for beta- and gamma-crystallin/glutathione mixed disulfide formation, inhibiting disulfide-crosslinked insolubilization. The data also implicate copper as a catalyst for the autoxidation of -SH groups in the lens, and suggest that alpha-crystallin chaperone-like activity may play a greater role in the lens nucleus than in the cortex in preventing oxidative insolubilization of crystallins.

Disruption of retinoblastoma protein family function by human papillomavirus type 16 E7 oncoprotein inhibits lens development in part through E2F-1

Complexes between the retinoblastoma protein (pRb) and the transcription factor E2F-1 are thought to be important for regulating cell proliferation. We have shown previously that the E7 oncoprotein from human papillomavirus type 16, dependent upon its binding to pRb proteins, induces proliferation, disrupts differentiation, and induces apoptosis when expressed in the differentiating, or fiber, cells of the ocular lenses in transgenic mice. Mice that carry a null mutation in E2F-1 do not exhibit any defects in proliferation and differentiation in the lens. By examining the lens phenotype in mice that express E7 on an E2F-1 null background, we now show genetic evidence that E7's ability to alter the fate of fiber cells is partially dependent on E2F-1. On the other hand, E2F-1 status does not affect E7-induced proliferation in the undifferentiated lens epithelium. These data provide genetic evidence that E2F-1, while dispensible for normal fiber cell differentiation, is one mediator of E7's activity in vivo and that the requirement for E2F-1 is context dependent. These data suggest that an important role for pRb-E2F-1 complex during fiber cell differentiation is to negatively regulate cell cycle progression, thereby allowing completion of the differentiation program to occur.

Protein associated with human lens 'native' membrane during aging and cataract formation

Plasma membrane contains extrinsic as well as intrinsic proteins. Changes in the extrinsic proteins of lens membrane during human aging and cataract formation have not been investigated in detail. Unlike previous studies which examined lens membrane after being stripped of extrinsic proteins by treatment with chaotropic agents, we have isolated whole or 'native' lens membrane on a sucrose gradient by ultracentrifugation of the total water-insoluble protein. Essentially all of the water-insoluble protein from young to aged to cataractous human lens appeared membrane associated. In young lens (20-37 years old), most of the membrane banded at the 25/45% sucrose interface fraction. This fraction contained relatively little urea-soluble protein and likely represents fiber-cell plasma membrane with its physiologically associated extrinsic and intrinsic proteins. With aging (62-80 years old), about one-third of the membrane, as judged by the distribution of cholesterol, banded at a much higher density (50/58% sucrose fraction). The higher density was due to a great increase in the membrane's relative protein content (protein/cholesterol). Although this extra protein was composed of both urea-insoluble and -soluble fractions, the urea-soluble protein predominated in all lenses. Cataractous lens differed from aged-clear lens in that much more of the total membrane (70-75%) had shifted to the high density and participated in this massive binding of cytosolic proteins. Although alpha-crystallin was the principal extrinsic-membrane protein in young lens, high molecular weight aggregate of modified (acidic) crystallins accounted for the increased extrinsic protein in aging. The extrinsic proteins bound to both clear-aged and cataractous lens membrane were aggregated. In conclusion, examination of human lens native membrane fractions revealed that the association of crystallins with membrane in both aging and cataracts was much greater than previously recognized and most of this increased protein was non-covalently bound to the membrane. Much more of the lens total membrane from cataractous than clear-aged lens was involved in this massive protein association and the protein bound to cataract membrane appeared more highly aggregated.

Degradation of native and oxidized beta- and gamma-crystallin using bovine lens epithelial cell and rabbit reticulocyte extracts

In many types of cells, modified proteins are selectively and rapidly removed by various proteolytic systems. In eye lens, as in most cells, there appears to be a multiplicity of proteolytic pathways, including ubiquitin-dependent, ATP-dependent and ATP-independent pathways. Each of these appears to be involved in the degradation of alpha-crystallins. The objective of this study was to determine if oxidized beta- and gamma-crystallins would be selectively degraded and which proteolytic systems might be involved. beta- and gamma-crystallins were oxidized by exposure to 137Cs radiation under N2O. This system generates *OH primarily. Oxidation of beta- and gamma-crystallins was indicated by decreased protein sulfhydryl content and tryptophan fluorescence, as well as by increased levels of carbonyl and high molecular weight aggregates with increasing radiation dose. gamma-crystallin was more susceptible than beta-crystallin to oxidation based on loss of native crystallin, increase in aggregates and fragmentation products, and loss of tryptophan. Low molecular mass polymers (dimers) appear to be the precursors of high mass aggregates induced upon oxidation. At a specific level of oxidative insult interchain covalent bonds in addition to disulfides were more extensive in the polymers of gamma-crystallin as compared to beta-crystallin. Except for beta-crystallin irradiated with 1 krad, the degradation rate of crystallins using both reticulocyte and bovine lens epithelial cells (BLEC) proteolytic systems increased in proportion to the extent of oxidation. Proteolysis of oxidized gamma-crystallins increased 1775% and 900%, respectively, using reticulocytes and BLEC supernatants as the source of proteases.(ABSTRACT TRUNCATED AT 250 WORDS)

On the composition and origin of the urea-soluble polypeptides of the U18666A cataract

The composition and origin of the urea soluble polypeptides which accumulate in the U18666A rat-cataract were studied. Chromatography on Sephacryl S-200 in 7.2 M urea separated the USP into 19-20 and 22-26 kDa enriched fractions. The polypeptide composition of these fractions was probed by immunoblotting of IEF and 2-D electrophoresis gels. The cataract USP largely focused at pHs comparable to alpha- and beta-crystallins. Immunoblotting of 2-D gels showed the USP to be composed predominantly of alpha- and beta-derived crystallins; little gamma-polypeptide was detected in the gels. Some of the insoluble alpha-crystallin appeared to be degraded. Changes in the lens WSP which accompanied the increase in USP were also measured. WSP decreased more than USP increased. Decreases in soluble high molecular weight proteins (alpha- plus beta-crystallins) and medium molecular weight proteins (beta-crystallins) were calculated which together could entirely account for the increased USP. An unexpected decrease in the lens soluble low molecular weight proteins (gamma-crystallins) appeared largely due to the selective leakage of gammas from the lens. The protein content of the ocular humors from eyes with cataracts increased 4 fold and contained polypeptides that focused on IEF like gamma-light crystallin and reacted with the gamma-crystallin antiserum. The cause of the protein insolubilization in the U18666A cataract is unknown but could be partially due to increased aggregation of alpha-crystallins secondary to loss of gamma-crystallins from the lens.

Modelling cortical cataractogenesis. XI. Vitamin C reduces gamma-crystallin leakage from lenses in diabetic rats

Normal and streptozotocin diabetic female Wistar rats were given vitamin C (VC) at 0.3% or 1.0% (w/w) in the diet: 1% dietary VC resulted, in 12-24 hr, in significant increases in serum ascorbate levels and lens ascorbate concentrations in normal rats. The increase was biphasic, with VC concentrations falling to a lower level which was still significantly elevated compared to controls in the period of 1.7-4 days for serum and 1.7-5 days for lenticular VC. At the end of 10 weeks the rats were examined for weight gain or loss, general body condition and cataracts. At the time of killing, blood was collected for measurement of serum glucose. Alpha-crystallin levels were determined in vitreous and aqueous humours using a radioimmunoassay. One lens from each rat was fixed for either scanning electron microscopy or light microscopy; the other lens was homogenized in 8 M guanidinium chloride for adenosine triphosphate analysis. In normal rats, a small amount of gamma-crystallin was found in the vitreous humour, and an even smaller amount in the aqueous humour. Diabetes caused a five-fold increase in the vitreous humour and a 2.5-fold increase in gamma-crystallin in the aqueous humour. Diabetes also led to a significant worsening in general body condition, loss of body weight, formation of cataracts, and decrease in lens adenosine triphosphate levels. Addition of VC to the diet of diabetic animals resulted in reduction in cataracts and a decrease of gamma-crystallin leakage into the aqueous and vitreous humours. VC had no effect on lens adenosine triphosphate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of an altered beta-crystallin with other proteins in the Philly mouse lens

An altered beta B2-crystallin is synthesized in the lens of the Philly mouse. This beta B2 has a more acidic isoelectric point than the beta B2 that is isolated from normal mouse lens. The altered beta B2 is immunologically reactive with antibody to the amino terminal of the beta B2-crystallin, but appears to be present in only very small quantities in the Philly lens. When the soluble proteins are isolated from the Philly lens and chromatographed by gel exclusion chromatography, the beta B2 can be found primarily in the heavy molecular weight fraction. Some immunoreactive material was also found throughout the higher molecular weight beta-crystallin region, beta H, and the lower molecular weight region, beta L. These results would indicate that the altered beta B2-crystallin in the Philly lens can interact with the other beta-crystallins in the lens; however, interactions of the beta B2-crystallin with the other proteins of the lens may cause rapid aggregation of the cellular proteins leading to the formation of the heavy molecular weight material. The increased number of these aggregates may eventually lead to the cataract formation in the Philly mouse.

UV laser photodamage to whole lenses

Lenses from rat or calf were exposed in vitro to UV radiation from a nitrogen laser operated at 337.1 nm or from an excimer laser operated at 3.8 nm. Visible light transmission was monitored during calf lens irradiations at 308 nm and found to decrease. Proteins were extracted from the irradiated rat or calf lenses, separated into water soluble and insoluble fractions, and analysed using SDS-PAGE. Comparison of these gels with dark controls showed that, following photolysis, there was loss of polypeptide material in the 20-30 kDa region and concomitant formation of polymers at 40 and 60 kDa, and at greater than 100 kDa in calf lens (308 nm irradiation) and rat lenses (337.1 nm irradiation) in vitro. In addition, there was evidence for formation of lower molecular weight polypeptides at 10 kDa in the protein from irradiated rat lenses. The rat SDS-PAGE gels were challenged against anti-calf gamma crystallin serum. There was clear evidence that the polymeric material, in the water insoluble protein fraction from the 337.1 nm photolyzed rat lenses was derived in part from gamma crystallin. The macromolecular changes detected in these photolyzed rat and calf lens proteins were similar to those previously reported to accompany aging in the human lens. Biochemical changes of the type observed in UV irradiated rat and calf lenses may be responsible for the loss of visible light transmission seen in calf lenses.

Light scattering and photocrosslinking in the calf lens crystallins gamma-II, III and IV

The calf lens proteins gamma-II, -III and -IV crystallin have been photolyzed in pH 7.5 phosphate buffer solution at 25 degrees C. The photolysis light source was either a xenon arc lamp/monochromator system set to pass 290 +/- 5 nm or a nitrogen laser operating at 337.1 nm. Photolysis experiments at 337.1 nm were done both in the presence and absence of added 1.0 x 10(-4) M N-formylkynurenine (NFK). In addition, 1 x 10(-5) M riboflavin was added as a photosensitizer in a few of the experiments. All solutions were 1.0 mg ml-1 protein, and 1.0 ml of solution was irradiated for periods ranging from 10 min to 3 hr. During the 337.1 nm irradiations, the turbidity of the protein solutions was continuously monitored using a He-Ne laser at 632.8 nm. Progress of the 290 nm irradiations was monitored by observing the loss of tryptophan fluorescence for each of the gamma crystallin proteins. The rate of growth of light scattering, upon 337.1 nm irradiation, was greatest for gamma-IV. Addition of NFK caused the rates of growth of UV-induced light scattering of all three gamma crystallins to increase significantly. These rates were in the order: gamma-IV much greater than gamma-III greater than gamma-II. Following UV exposure, the protein solutions were analyzed using UV-visible absorption spectroscopy and SDS-PAGE. Irradiated gamma crystallin solutions showed increased optical density throughout the visible region, resulting from solution turbidity.(ABSTRACT TRUNCATED AT 250 WORDS)

The murine cataractogenic mutation, Cat Fraser, segregates independently of the gamma crystallin genes

The murine mutation, Cat Fraser (CatFr), causes dominantly inherited ocular cataracts. Lenses of adult mice bearing this mutation contain reduced amounts of all seven γ-crystallin proteins and their corresponding transcripts. Levels of other lens proteins and transcripts appear normal and no extra-ocular effects of the mutation have been observed. The selective effect of this mutation on the γ-crystallins is consistent with the possibility that the site at which it occurs is involved in the coordinated regulation of the family of genes which encodes them. We have shown that several restriction fragment length polymorphisms in the γ-crystallin genes segregate independently of the CatFr mutation. Therefore, despite its selective effect on the expression of the γ-crystallin genes, the mutation is not linked to them. This observation rules out the possibility that the mutation is in a cis-acting regulatory site.

Solubilization of the lens water-insoluble fraction by sonication

A method is reported whereby the solubilization of the bulk of the lens water-insoluble fraction is accomplished by a short sonication of the suspended proteins in low salt buffers. This procedure solubilized greater than 90% of a bovine lens water-insoluble fraction and 80% of the normal human lens water-insoluble fraction. Decreased protein was solubilized from cataractous lenses, but in every case sonication was at least equivalent to extraction with 6.0 M urea. Fractionation of the solubilized proteins by Agarose A-1.5 m gel filtration chromatography showed native molecular weights for bovine lens, but only partial disaggregation with human lens extracts. A sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) comparison of the proteins solubilized by sonication and 6.0 M urea extraction showed no major differences except that sonication solubilized more of the highly cross-linked protein which remained at the top of the gel.

The Emory mouse cataract: changes in the beta and gamma-crystallins during aging and cataractogenesis as revealed by isoelectric focusing of the native soluble proteins

Isoelectric focusing on ultra-thin polyacrylamide gels was employed to study alterations in the distribution patterns of soluble beta and gamma-crystallins during the development of cataract in the Emory (EM) mouse. These alterations were compared with corresponding changes occurring in clear control lenses of the same age from the cataract-resistant (CR) strain. At two months of age both strains gave similar patterns. At four and fourteen months the EM lens showed depressed beta peaks compared to the CR lens. At fourteen months the EM lens had reduced gamma 2 and gamma 5 compared with the CR lens. At twenty-four months the EM lens had all gamma's markedly reduced, especially gamma 5, with the complete disappearance of gamma 4 which was however still present in the CR lens at twenty-six months. At twenty-four months the EM lens had a relatively large amount of beta 5 but no beta 5a or beta 5b; the latter two were present in the CR lens surrounding beta 5 which was less prominent than in the EM lens. The decrease in gamma's in the twenty-four months EM lens was approximately equalled by an apparent increase in beta's. Of the above changes only the decrease in beta's at four months can be considered precataractous and perhaps a primary change in cataractogenesis. The later changes in both beta and gamma may be secondary although they are certainly at least associated with cataractogenesis. The invariable decrease in gamma with age is noticeably accelerated in cataractogenesis.

Calcium-induced high molecular weight proteins in the intact rabbit lens

We have investigated the ability of Ca2+ to induce the formation of high molecular weight (HMW) proteins in the intact lens. Ca2+ cataracts were produced in rabbit lenses by culturing the lenses for either four days in medium containing 20 mM Ca2+ or for three days in medium containing 100 mM Ca2+. Lenses cultured in 20 and 100 mM Ca2+ medium became opaque after 20 hr and contained 30 and 200 times higher levels of Ca2+, respectively, than transparent lenses cultured in medium containing 1 mM Ca2+. Lenses exposed to 100 mM Mg2+ did not lose transparency. The opacification of the lenses extended to a depth of 1 mm into the cortical layer and did not involve the nucleus. No significant differences were found in the concentrations of either soluble or insoluble proteins present in freshly excised lenses and Ca2+ cataracts. Soluble HMW proteins, greater than 1.5 X 10(6) daltons, were in two- and five-fold greater amounts in the 20 and 100 mM Ca2+ cataracts, respectively, compared to controls. HMW protein present in the 100 mM Ca2+ cataract amounted to approximately 3% of the total soluble protein in the lens. The amount of Ca2+ present in the HMW fraction was 1 Ca2+ per 5 X 10(5) daltons, no higher than that present in the unaggregated crystallins. No evidence was found for covalent bonding in the aggregate. Results of polyacrylamide gel electrophoresis and double immunodiffusion indicated the presence of alpha- and beta- but not gamma-crystallin in the HMW protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary restriction retards age-related loss of gamma crystallins in the mouse lens

The soluble crystallins in lenses from diet-restricted and control mice of diverse ages (2, 11, or 30 months) were studied by high-performance liquid chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Results obtained with both methods suggest that dietary restriction decelerates age-related loss of soluble gamma crystallins.

Immunohistochemical studies of lens crystallins in the dysgenetic lens (dyl) mutant mice

The lens in the dyl mutant mice shows a persistent lens-ectodermal connection as well as degeneration and extrusion of lens materials after the initial differentiation of the fibres. Immunohistochemical investigation of the ontogeny of the lens crystallins in this developing mutant lens has been carried out using the indirect immunofluorescence staining method with antiserum to adult mouse lens total soluble proteins. The results have been compared with those for coisogenic normal lens used as a control. In both, the first positive reaction was detectable at identical stages of lens development. A rapid increase in the intensity of fluorescence, most marked in the elongating fibre progressing through the equatorial region to the epithelium, was recorded in the mutant as well as in the normal lens. However, the stalk leading to the lens epithelium did not show any reaction. Appearance of vacuoles in the lens nucleus and cortex marked the beginning of degeneration of fibres which otherwise showed strong fluorescence. This was followed by extrusion of lens crystallin materials through the stalk. As a result, the lens became increasingly reduced and malformed but the surviving cells making up the vestigeal lens in the adult showed positive immunofluorescence. The results demonstrate that despite a failure of lens-ectoderm separation in the mutant mice, the ontogeny of the lens crystallins and differentiation of the lens up to a certain stage of development follow an apparently normal course before the commencement of cataractous degeneration.

Physicochemical characterization of high-molecular-weight alpha-crystallin subpopulations from the calf lens nucleus

Calf lens nuclear alpha-crystallin was separated into five molecular weight subpopulations by exclusion chromatography on Bio-Gel A-5m. These subpopulations were compared by amino acid analysis, ultraviolet absorption analysis, fluorescence, far- and near-ultraviolet circular dichroism, isoelectric focusing, SDS-polyacrylamide gel electrophoresis and sedimentation velocity analysis. Although only minor differences were detectable in most physicochemical properties, progressive changes were found in the near-ultraviolet circular dichroism spectra and in pellet hardness after centrifugation. Minute amounts of beta-crystallin polypeptides and a 43 kDa component were present in all five subpopulations. In addition, the highest molecular weight aggregates contain some gamma-crystallin polypeptides. A slow re-equilibration of separated subpopulations towards the initial distribution was observed by rechromatography.

Abnormalities of crystallins in the lens of the CatFraser mouse

We have analysed, by column chromatography and two-dimensional electrophoresis, the soluble proteins present in the lenses of normal mice and of mice heterozygous (Cat/+) and homozygous (Cat/Cat) for the CatFraser mutation which causes a dominantly inherited cataract. In Cat/+ and Cat/Cat lenses, the gamma-crystallins comprise a smaller fraction, and the alpha-crystallins a greater fraction, of the total crystallins present than found in normal lenses. These changes in composition involve all the subunits of each crystallin class and show a dosage effect, the change being greater in Cat/Cat than Cat/+ lenses. In both Cat/+ and Cat/Cat lenses, the beta H-crystallin aggregate is lost and subunits are present which are not detectable in normal lenses.

Studies on lens proteins of mice with hereditary cataract. I. Comparative studies on the chemical and immunochemical properties of the soluble proteins of cataractous and normal mouse lenses

Total soluble and insoluble proteins of the lens were similar in normal and hereditary cataractous mice up to 1 week of age. Thereafter, the normal mouse lens showed a continued increase in weight and protein content until 500 days of age. In cataractous mice, while the total protein content increased up to 60 days and reached a plateau, the soluble protein content declined dramatically from day 22 to day 60, and then the rate of decrease remained constant up to 500 days. At different ages, the soluble proteins were separated by gel filtration into the high molecular weight proteins, alpha-, beta- and gamma-crystallin fractions. All of these showed an age-related increase in the normal lens, and the relative values of alpha- and beta-crystallins increased for a 410-day period. On the other hand, in the cataractous process, the high molecular weight protein increased, and alpha-, beta- and gamma-crystallins decreased: the degree was especially marked in gamma-crystallin. Immunochemical studies indicated that the aggregation of beta-crystallin occurred much earlier in the cataractous lens than in the normal. Analysis of the amino acid composition and ultraviolet absorption spectra revealed no significant chemical differences between the crystallins of the normal and the cataractous lens.