Differential metabolism and leakage of protein in an inherited cataract and a normal lens cultured with ouabain

Failure of Oxysterols Such as Lanosterol to Restore Lens Clarity from Cataracts

The paradigm that cataracts are irreversible and that vision from cataracts can only be restored through surgery has recently been challenged by reports that oxysterols such as lanosterol and 25-hydroxycholesterol can restore vision by binding to αB-crystallin chaperone protein to dissolve or disaggregate lenticular opacities. To confirm this premise, in vitro rat lens studies along with human lens protein solubilization studies were conducted. Cataracts were induced in viable rat lenses cultured for 48 hours in TC-199 bicarbonate media through physical trauma, 10 mM ouabain as Na+/K+ ATPase ion transport inhibitor, or 1 mM of an experimental compound that induces water influx into the lens. Subsequent 48-hour incubation with 15 mM of lanosterol liposomes failed to either reverse these lens opacities or prevent the further progression of cataracts to the nuclear stage. Similarly, 3-day incubation of 47-year old human lenses in media containing 0.20 mM lanosterol or 60-year-old human lenses in 0.25 and 0.50 mM 25-hydroxycholesterol failed to increase the levels of soluble lens proteins or decrease the levels of insoluble lens proteins. These binding studies were followed up with in silico binding studies of lanosterol, 25-hydroxycholesterol, and ATP as a control to two wild type (2WJ7 and 2KLR) and one R120G mutant (2Y1Z) αB-crystallins using standard MOE^TM (Molecular Operating Environment) and Schrödinger's Maestro software. Results confirmed that compared to ATP, both oxysterols failed to reach the acceptable threshold binding scores for good predictive binding to the αB-crystallins. In summary, all three studies failed to provide evidence that lanosterol or 25-hydroxycholesterol have either anti-cataractogenic activity or bind aggregated lens protein to dissolve cataracts.

Calcium and the physiology of cataract

Calcium has long been known to play a role in cataract formation but techniques have only recently become available for investigating the physiological mechanisms. Previous studies showed that lens membrane permeability alters when the external calcium concentration falls below 1 mM, so it was interesting that values for human aqueous from cataract patients ranged from 0.45 to 2.0 mM. The mean value for the aqueous was one half that for the plasma. The calcium concentration in cataractous lenses ranged from 0.1 to 64 mM and lenses with a high calcium concentration also had a high sodium content. In lenses with near normal sodium content the highest calcium concentrations were associated with highly localized opacities, while nuclear cataracts had a low calcium content. The relationship between calcium and transparency was investigated in a rat lens system using ion-sensitive microelectrodes. The distribution of free calcium in the lens varied with age and was correlated with a change in the sensitivity of the lens to cold cataract and a change in lens birefringence. The highest free calcium levels were obtained from lenses incubated in 10 mM-calcium in the absence of glucose and these lenses showed most light scattering. Ion-sensitive microelectrode techniques applied to human lenses yielded calcium levels of 0.1 microM-2 mM. In lenses with dense, highly localized opacities the calcium distribution was not uniform and was highest in regions that scattered most light. The movement of calcium through individual membrane channels was investigated using patch clamp techniques. Three types of ionic channels have been identified in the lens. The smallest appears to be a calcium channel; the larger current fluctuations are associated with sodium and potassium movements. In organ culture studies of the bovine lens, a marked decrease in protein synthesis and net leakage of proteins was associated more strongly with an increase in calcium than with an increase in sodium. The stability of the lens protein gel thus seems to depend on maintaining a low internal level of calcium ions.

HSF4 is required for normal cell growth and differentiation during mouse lens development

The heat shock transcription factor (HSF) family consists of three members in mammals and regulates expression of heat shock genes via a heat shock element. HSF1 and HSF2 are required for some developmental processes, but it is unclear how they regulate these processes. To elucidate the mechanisms of developmental regulation by HSFs, we generated mice in which the HSF4 gene is mutated. HSF4-null mice had cataract with abnormal lens fiber cells containing inclusion-like structures, probably due to decreased expression of gamma-crystallin, which maintains protein stability. Furthermore, we found increased proliferation and premature differentiation of the mutant lens epithelial cells, which is associated with increased expression of growth factors, FGF-1, FGF-4, and FGF-7. Unexpectedly, HSF1 competed with HSF4 for the expression of FGFs not only in the lens but also in other tissues. These findings reveal the lens-specific role of HSF4, which activates gamma-crystallin genes, and also indicate that HSF1 and HSF4 are involved in regulating expression of growth factor genes, which are essential for cell growth and differentiation.

Peptidases play an important role in cataractogenesis: an immunohistochemical study on lenses derived from Shumiya cataract rats

The role of proteolytic enzymes in Shumiya cataract rats in alterations to lens proteins during cataract formation was studied immunohistochemically using antibodies against exopeptidases, such as lysosomal dipeptidyl peptidase II (DPP II), cytosolic dipeptidyl peptidase III, and soluble and membrane-bound alanyl aminopeptidases, and against cytosolic endopeptidases such as mu- and m-calpains, and 20S proteasome. AlphaB-crystallin was detected as a proteolytic marker in the lenses. A constant immunoreactivity against all the antibodies employed was observed in the lens epithelium independent of the strain and age of the rats. A weak immunoreactivity against exo- and endopeptidases and an intense reactivity against alphaB-crystallin were observed in the lens fibres of control rats at all ages. The immunoreactivity of these peptidases in lens fibres increased with age in cataract rats, but that of alphaB-crystallin decreased. No reactivity against exo- and endopeptidases was seen in the perinuclear region of lenses of control rats at all ages or in Shumiya cataract rats at 8 and 10 weeks of age, but an intense reactivity against these peptidases was observed in the lens perinuclear region of lenses in cataract rats at 12 and 14 weeks of age. AlphaB-crystallin immunoreactivity was observed with ordered striations in the lens perinuclear region of all control rats whereas the striations in this area of cataract rat lens were disorganized. Membrane-bound alanyl aminopeptidase was detected feebly in the lens epithelium and fibres of both types of rat at all weeks of age. These findings indicate that exo- and endopeptidases, except for membrane-bound alanyl aminopeptidase, are expressed intensively and are age-dependent. Conversely, the amount of alphaB-crystallin decreased with age in lens fibres of cataract rats. Calpains (mu- and m-), 20S proteasome, dipeptidyl peptidases II and III and soluble alanyl aminopeptidase are thought to induce lens opacification kinetically during cataract formation in Shumiya cataract rats through the intracellular turnover of lens proteins.

Cataractogenesis in transgenic mice containing the HIV-1 protease linked to the lens alpha A-crystallin promoter

Several lines of transgenic mice were generated with either active or inactive forms of the human immunodeficiency virus type 1 (HIV-1) protease gene under the control of the mouse lens alpha A-crystallin promoter. Mice bearing the inactive protease coding sequence displayed no gross abnormalities in the lens, while mice with the active protease developed time-dependent bilateral cataracts. One line, TG61, developed cataracts in utero while the second line, TG72, developed cataracts postnatally. TG61 mice, homozygous for the transgene, developed severe microphthalmia and were significantly smaller than the control mice at postnatal day 30. two-dimensional-polyacrylamide gel electrophoresis analysis of the protein profiles of TG72 and TG61 lenses revealed extensive modifications in the lens crystallins. Proteolysis in the homozygous TG72 mouse lenses began at postnatal day 20 with the disappearance or partial loss of beta B1-, beta B3-, and beta A3-crystallins and the appearance of crystallin fragments. Protein leakage and the gradual breakdown of cytoskeletal elements also occurred. In contrast, the opacification of the homozygous TG61 lenses appeared to have been influenced by differentiation and developmental processes. It appears that HIV-1 protease expression activates other proteases, and these enzymes, in concert with HIV-1 protease, are responsible for the protein modifications that eventually result in the opacification of the lens.

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.

Cataract incidence and analysis of lens crystallins in the water-, urea- and SDS-soluble fractions of Emory mice fed a diet restricted by 40% in calories

Restriction of dietary calorie intake is associated with life extension and with the delay of age-related disorders. Preliminary studies demonstrated that by feeding the Emory mouse a diet restricted by 21% in calories cataract and insolubilization of protein could also be delayed. To observe the effects of calorie restriction over prolonged portions of adulthood, Emory mice were fed the control diet (C) or a diet restricted by 40% in calories (R). Feeding the R diet was associated with delayed formation or progress of cataract over virtually the entire second half of life. At 11 months of age, bilateral grade 5 cataracts were present in 17% and 2% of C and R lenses, respectively. At 22 months of age, bilateral grade 5 cataracts were present in 90% and 18% of C and R lenses, respectively. The distribution of alpha-, beta-, and gamma- crystallins in the water-soluble, urea-soluble, and SDS-soluble fractions indicates more similarities than differences between C and R lenses with a specific grade of cataract or of a given age. However, there were significant and abrupt (after grade 4 cataract) losses of particular gamma-crystallins; gamma-crystallins which were not prominent at earlier stages became the major gamma-crystallin moieties. Losses of alpha-crystallins were also noted upon cataract formation or aging in most of the fractions. Aggregates including gamma- and alpha-crystallins also accumulate faster in the C group.

Progressive modifications of mouse lens crystallins in cataracts induced by buthionine sulfoximine

L-buthionine-S,R-sulfoximine (BSO), a specific inhibitor of GSH biosynthesis, was administered four times daily to mouse pups on post-natal days 7 and 8, inducing initiation of opacification on day 9. The initial progression of the cataract (less than 24 hr) was divided into four stages: (1) developing floriform; (2) mature floriform; (3) degenerate floriform; and (4) amorphous translucent cataract. Following this, dense corticonuclear opacities developed within several days. Two-dimensional gel electrophoresis of water-soluble whole lens extracts indicated that the most rapid early cataractous changes, occurring mainly during stage 2, were loss of the two major components of the heavy beta-crystallin fraction, a 31-kDa basic polypeptide and an acidic component at 27 kDa, concomitant with the appearance of new species at 30 and 25 kDa. This was followed by more extensive modification of both alpha and beta-crystallins during stages 3 and 4 and the appearance of abnormal species at 26, 19 and 18 kDa, which were slightly more acidic than the major normal alpha A-crystallin polypeptide. The gamma-crystallin components, relatively unaffected at stage 4, were then lost rapidly as dense opacities ensued. By contrast with the water-soluble fraction, the normal day 9 urea-soluble fraction was deficient in gamma-crystallin polypeptides and enriched in anodic components whose relative electrophoretic mobilities were similar to those reported previously for phosphorylated bovine alpha A-crystallin and several cytoskeletal polypeptides. At stage 4 of the cataract, the modifications of normal alpha and beta-crystallin components in the urea-soluble fraction paralleled those in the water-soluble fraction, but the products seen were more numerous. In addition, the cytoskeletal proteins were no longer detectable. Substantial increases in lens Ca2+ that precede all of the above changes in lens polypeptide composition suggest that Ca(2+)-activated proteolysis may play a major role in development of BSO cataracts.

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.

Molecular biology: recent studies on enzyme/crystallins and alpha-crystallin gene expression

Jin H. Kinoshita has had a major role in fostering investigations on the molecular biology of the eye during his tenure as Scientific Director of the National Eye Institute (N.E.I.). His appreciation and support of molecular biology have been the basis for many advances in this area in the N.E.I. and have led to a fertile breeding ground for molecular studies on the normal and diseased eye. Our recent studies have shown that the taxon-specific crystallins have been recruited from numerous metabolic enzymes; they are expressed in the lens as abundant structural proteins and in other tissues as enzymes, a situation we call gene sharing. Although not taxon-specific, alpha B-crystallin, structurally related to the small heat-shock proteins, is expressed in diverse tissues; by contrast, alpha A-crystallin appears to be lens-specific. Recombinant DNA studies indicate that the two alpha-crystallin genes use different promoter sequences and trans factors for their expression. Thus, the ubiquitous alpha-crystallin genes may provide some insight into the mechanisms used to recruit metabolic enzymes as taxon-specific lens crystallins.

Modeling cortical cataractogenesis: IX. Activity of vitamin E and esters in preventing cataracts and gamma-crystallin leakage from lenses in diabetic rats

Normal and streptozotocin diabetic female Wistar rats were given vitamin E in the diet as the tocopherol, acetate, or succinate form (2,850 IU/kg food). At the end of 6 weeks, the rats were examined for weight gain or loss, general body condition, and cataracts. At sacrifice, blood was collected for measurement of serum glucose, and gamma-crystallin levels were measured in aqueous and vitreous humors using a radioimmunoassay. One lens was homogenized in 8 M guanidinium chloride for ATP analysis. In normal rats, gamma-crystallin was detected in both aqueous and vitreous humors, with the higher concentration in the vitreous humor. Diabetes caused a sixfold increase in gamma-crystallin in both the aqueous and vitreous humors. Diabetes also led to a significant worsening in general body condition, loss of body weight, formation of cataracts, and decrease in lens ATP levels. Addition of vitamin E and vitamin E succinate, but not vitamin E acetate, to the diet resulted in reduction of gamma-crystallin leakage into the vitreous humors and an increase in body weight. There was no improvement noted for the lens ATP levels, the general body condition, or visual cataract score. Neither streptozotocin-induced diabetes nor vitamin E in the diet appeared to affect the weight of the lenses.

Lens crystallins in aqueous and vitreous humor in selenite overdose cataract

The purpose of this research was to determine if crystallins were released from the lens into the aqueous and vitreous humor in selenite-overdose cataract. At 20 days post injection, aqueous and vitreous were obtained from lenses containing nuclear cataract or "mature" cataract (nuclear and cortical opacities). Humors were analyzed by direct ELISA and immunoblotting using antibodies against lens crystallins. Both types of selenite cataract showed increased alpha-, beta- and gamma-crystallins in aqueous and vitreous. Increases in crystallins in vitreous humor were larger than in aqueous humor, possibly because of slower turnover of vitreous. Extensive membrane damage in selenite cataract probably caused leakage of crystallins from the lens to aqueous and vitreous humors. This model may be useful for studies on the immunologic response of the body to lens crystallins released by cataract.

The effects of aging and cataract formation on the trypsin inhibitor activity of human lens

Assays were carried out to determine the trypsin inhibitor activity present in the water-soluble and water-insoluble fractions of human lenses of various ages. Little change was seen in the inhibitor activity of the water-soluble protein fraction. When this fraction was chromatographed on an Agarose A-1.5 m column, however, the inhibitor activity was increasingly associated with the high molecular weight (HMW) protein fraction with age. A gradual increase in water-insoluble inhibitor was seen up to age 60, which correlated with the increase in protein in this fraction. After age 60, a marked increase in the water-soluble inhibitor activity was observed. In 80-90-yr old lenses, 1 mg of water-insoluble protein was able to inhibit 200 micrograms of crystallin trypsin by 50%. Similar assays on a collection of cortical and brunescent cataracts also showed very high levels of water-insoluble inhibitor activity. In most cases, these values were higher than those for the age-matched control lenses. Fractionation of the water-insoluble proteins showed that the bulk of the activity remained with the urea-insoluble fraction in cataractous lenses. A low molecular weight trypsin inhibitor was isolated from the water-soluble and water-insoluble fractions of human lenses. An age-dependent increase in this inhibitor was observed by activity measurements and electrophoretic analysis.

Suppression of lens gamma-crystallin encoded by Len-1 in the mutant mouse with eye lens obsolescence (Elo)

Eye lens obsolescence (Elo) is a heritable, autosomal dominant eye disorder in mice. The Elo mutation lies close to the Len-1 locus on mouse chromosome 1, and it was speculated that the Elo locus affects the expression level of gamma-crystallin. To clarify this, we used the BASO-DALT system to compare the amount of lens gamma-crystallin between Elo and normal mice. The lens extracts from 10-day-old Elo mice revealed the same profiles and expression levels of alpha- and beta-crystallins as those of +/+ mice. Among gamma-crystallins, the amounts of the components encoded by Len-1 were lower than wild-type levels, but that encoded by Len-2 was not in the Elo mouse. NaDodSO4-gel analysis revealed that the lower gamma-crystallin level could be detected as early as the 14-day stage in the lens of the Elo mouse embryo. The suppression of gamma-crystallin gene expression in Elo mice was examined at the mRNA level. The concentration of gamma 2-crystallin mRNA in Elo mice lens was estimated at 1/13 of that of normal lens, indicating that a small amount of transcript production from gamma-crystallin genes may cause the low level of gamma-crystallin in Elo lens.

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.

Rat lens prostaglandin biosynthesis during galactose-induced cataractogenesis

The relationship between the development of galactose-induced cataractogenesis and rat lens microsomal prostaglandin (PG) biosynthesis was studied. Within 24 hr of the introduction of 50% galactose to the rat's diet, lens PGF2 alpha production fell dramatically to 31% of control. Following the initial depression of PGF2 alpha biosynthesis, the ability to generate PGF2 alpha in lens microsomes slightly recovered reaching 58- and 53% of controls at day 2 and day 5 on the sugar diet, respectively. Determination of microsomal PGF2 alpha biosynthesis at 9- and 21 days revealed a continued decline in PG synthesis with complete cessation of PGF2 alpha synthesis by day 36 (hypermature cataracts, +5). The decreased PGF2 alpha biosynthetic capacity was a result of decreased cyclo-oxygenase activity since: PGE2 production demonstrated a similar time course for inhibition; and lens microsomes from control and galactose fed rats revealed no difference in the PGs produced from PGH2 endoperoxide. Neither galactose (1 mM) nor galactitol (1 mM), when added to control microsomal preparations inhibited PG biosynthesis, eliminating the possibility of a direct effect of the sugar, or its metabolite, on cyclo-oxygenase activity. While PG biosynthesis was rapidly inhibited by the galactose feeding no changes were observed in basal lens cyclic AMP levels measured during the first 5 days of the feedings. These results demonstrate that depressed PG biosynthesis is an early consequence of galactose feeding.

Calcium-induced opacification and loss of protein in the organ-cultured bovine lens

A long-term system of organ culture for bovine lenses was used to investigate the effect of osmotic stress on lens opacification and crystallin loss. Lenses were pre-incubated in control medium containing L-[U-14C]tyrosine so that labelled crystallins were produced. The fate of these crystallins was studied in relation to two forms of osmotic stress. The addition of either ouabain or EGTA to the medium induced severe osmotic swelling and disturbance of the lens monovalent cation balance, but only the former treatment was followed by an increase in lens calcium. The changes due to osmotic stress were accompanied by loss of transparency and protein only in the lenses with increased calcium. Both opacification and increased calcium were found largely to be confined to the outer cortical fibres. Protein loss increased with time as lens calcium continued to increase. The protein recovered from the incubation medium was characterized by gel filtration and immunological techniques. The first protein detected was beta L-crystallin, and this formed the major part of the lost protein throughout, although alpha- and gamma-crystallins were detected at a later stage. Increased calcium also resulted in a change in the susceptibility of the crystallins to aggregation, since there was an increase in [14C]tyrosine incorporated into the lens high-molecular-weight (HM) fraction after exposure to ouabain, but not after exposure to EGTA. The relevance of these findings to human cataract is discussed.

Sodium-23 magnetic resonance imaging of the eye and lens

In order to develop a better understanding of cataract and to evaluate the effectiveness of potential drugs, noninvasive techniques must be devised to detect early metabolic changes. As a prelude to these goals, sodium-23 imaging experiments operating at 29.8 MHz (2.7 teslas) were performed on the bovine eye and lens. A spatially localized transverse relaxation time (T2)-weighted spin-density map of the sodium-23 within the lens is presented, with a resolution better than 250 micron. Due to the presence of short-T2 (3 msec) components within the lens, only the use of the planar-integral projection reconstruction (PPR) imaging scheme allowed sufficiently short echo-times (1 msec) to permit sodium-23 signal detection. These noninvasive imaging results show differences in the apparent sodium concentration within the lens that are consistent with separate, invasive measurements of sodium concentration. Separate analysis (with no spatial localization) at 79.4 MHz (7.2 teslas), using a shift reagent (dysprosium) to distinguish extracellular from intracellular sodium, indicates that approximately 62% of the detected sodium-23 signal is intracellular. These results are consistent with observations based on invasive measurements and further support the existence of the pump-leak system and a sodium gradient within the lens.

Morphological and cell volume changes in the rat lens during the formation of radiation cataracts

Earlier studies in our laboratory showed that 24 hr after X-irradiation, epithelial cells of early postnatal rat lenses increased in volume. Three days after X-irradiation, the underlying lens fibers increased in volume. This finding suggested a correlation between damage to epithelial cell volume regulation and subsequent fiber cell swelling. To test this hypothesis 4-week-old rat lenses were three-dimensionally reconstructed to determine average cell volumes of specific lens regions and wet weights of whole lenses were measured during radiation cataract formation. In addition, the differentiation of epithelial cells into lens fibers was monitored by autoradiography. Four-week-old Sprague-Dawley rats were injected with [3H]-thymidine and, 24 hr later, their eyes were irradiated with either 400 or 1200 rad. Lenses were examined with a slit lamp and cataracts were graded on a scale of 1+ to 4+. Animals were killed 24 hr and 3, 5, 15 and 30 weeks after exposure. Lenses were serially sectioned at 0.75 micron and epithelial, equatorial and cortical fiber cell volumes were determined. Rats exposed to 400 or 1200 rad developed 0.5-1.5+ or 2.5-3.0+ cataracts, respectively, 10-16 weeks after X-irradiation. Epithelial and equatorial cells of both groups did not significantly increase in volume during this period. Three weeks after irradiation with 1200 rad cortical fibers were disorganized and had increased volume. By 5 and 15 weeks, cortical fibers had more normal cell volumes, although their morphology remained grossly altered. Cortical fiber volume of lenses irradiated with 400 rad were not significantly different from control lenses throughout the experimental period. By 15 weeks lenses irradiated with 400 rad showed subtle changes in morphology. Wet weight determinations indicated that the localized increase in cortical fiber volume did not result in an increase in the wet weight of the entire lens. Autoradiography showed that affected fibers had been epithelial cells at the time of X-irradiation. These results provide additional evidence that disturbances in fiber differentiation are involved with cataract formation, but do not support the initial hypothesis that a disturbance in epithelial cell volume regulation leads to fiber cell swelling. Earlier results suggesting defects in lens epithelial volume regulation in radiation cataract formation may have been complicated by ocular inflammation.

A temporal and spatial study of the synthesis and degradation of water-soluble and insoluble proteins in galactosemic rat lenses

Events of synthesis and degradation of water-soluble and -insoluble rat lens proteins were examined during a 21-day period of galactose-induced cataractogenesis. Epithelial cells continued to synthesize both fractions of proteins throughout the duration of the experiment. There were few qualitative changes in the pattern of proteins expressed and the rate of synthesis did not decrease. In contrast, synthesis of both water-soluble and -insoluble proteins by the cortical cells decreased and then became totally inhibited indicating no selective or differential inhibition of synthesis of one protein fraction, but that both water-soluble and -insoluble proteins in the cortical cells are equally affected by the metabolic changes associated with galactose-induced cataractogenesis. The cells within the nuclear region of control lenses were capable of incorporating [35S]-methionine into protein. This synthesis was completely abolished in 7- and 14-day nuclear cells. Labeled proteins were found to be present in the 21-day nuclear preparation to a level almost three times greater than that observed in the control nucleus. Degradation was minimal in both fractions of epithelial proteins, whereas selective degradation of proteins from both fractions was apparent in 14- and 21-day cortical cells. At day 14, the water-soluble protein profile from nuclear cells became less complex with the loss of several major bands, but these bands as well as some unique bands appeared at day 21. The profile of water-insoluble nuclear proteins remained unchanged over 21 days.

Characterization of lens proteins. IV. Analysis of soluble high molecular weight protein aggregates in human lenses

Soluble proteins of individual human normal and nuclear cataractous lenses of 60-70-year-old subjects were collected for this investigation. The average wet weights of both the normal and nuclear cataractous lenses were found to be essentially identical, approximately 230 mg. When the lens proteins, either cortical or nuclear, were subjected to fractionation by Sephadex G-200 sf chromatography, six fractions (F-I to F-VI) were obtained and their respective molecular weights approximated. F-I, which contains alpha-crystallin and high molecular weight aggregates, was subsequently fractionated through a series of Bio-gel A chromatographic columns. The quantities of the proteins and the molecular weights of each fraction were obtained. All the proteins fractionated were subjected to SDS gel electrophoresis by which the molecular weights of the subunits were obtained. The distribution and molecular weights of proteins smaller than 0.2 X 10(6) showed certain changes, more noticeable in the nucleus than in the cortex, between the normal lens and nuclear cataractous lens. For the high-molecular-weight protein aggregates, the major fraction in the lens cortex was found to be in the 5-1.5 X 10(6) range, representing 10 and 12% of the total protein in the normal and cataractous lenses, respectively. The major fraction in the nucleus was found to be greater than 150 X 10(6), representing 11 and 19% for the normal and cataractous lenses respectively. The above data are presented for the first time to show the differences in distribution of the high-molecular-weight proteins in the cortical and nuclear regions, and their respective changes in cataractogenesis. Based upon these data, we are able to calculate the average molecular weights of (1) the soluble cortical and nuclear proteins and (2) the total soluble protein, in the normal and cataractous human lenses.

Effect of 8-methoxypsoralen on rat lens cations, membrane potential and protein levels

Systemic application of 8-methoxypsoralen to rats, followed by u.v.-irradiation, induces a minor change in the lens membrane potential after one week, and by this time marked histological changes have already occurred. Alterations in lens sodium and potassium concentrations followed these early changes and coincided with the appearance of small light-scattering vacuoles when the lenses were examined in vivo with a slit lamp. Dramatic changes in the dry weight, soluble protein and calcium content of the lenses were apparent only after eight weeks from the start of treatment, and these changes coincided with the appearance of very large vacuoles in slit-lamp examinations of the eye.

The Emory mouse cataract: accumulation of free leucine and incorporation into protein by cataractous and clear lenses

Leucine metabolism has been studied in the Emory mouse lens to determine if there is a correlation between the rates of leucine accumulation and incorporation and the stage of cataractogenesis. Developing cataracts were graded in vitro according to severity and then incubated in tracer leucine. There was no significant difference between grades with respect to leucine incorporation or accumulation of free leucine, although there was a gradual decrease in CL/CM as the cataracts increased in severity. The results showed a high degree of variability, especially for incorporation, leading to the conclusion that the morphological appearance of these cataracts is not predictive of their metabolic behavior as revealed by leucine accumulation or incorporation.

Age-related and distributional changes in the trypsin inhibitor activity of bovine lens

Age-related changes in the bovine lens trypsin inhibitor activity were measured by assaying water soluble extracts of 10 concentric slices from the periphery to the center of the lens. Inhibitor assays were carried out at pH 7.0 and 7.9 using prenatal, calf and mature lenses. The inhibitor at pH 7.0 remained constant throughout the lens but the pH 7.9 activity decreased sharply in the lens nucleus. This was particularly true for the prenatal and calf lenses. Agarose A-15 m gel filtration of the water soluble inhibitor activity showed a decrease in inhibitor in the alpha-crystallin region and a corresponding increase in inhibitor activity in the HMW protein peak. Inhibitor assays were carried out on the water insoluble fractions following solubilization in 6.0 M-urea. Little or no inhibitor activity was seen in the outer cortical fractions but the inner cortex and nucleus contained high levels of inhibitor activity in the water insoluble fraction with specific activities 7- to 10-fold higher than the comparable crude lens extracts. These data suggest that the lens inhibitor activity at pH 7.0 and 7.9 aggregate into a HMW complex and with time preferentially enter the water insoluble fraction. The distribution of a purified 5.5 K inhibitor protein between the water soluble and the water insoluble fraction was measured. In the periphery all of this inhibitor was in the water soluble fraction, but toward the center of the lens this inhibitor began shifting to the water insoluble fraction. Slices taken from the lens nuclear region showed that all the inhibitor was in the water insoluble fraction as detected by both activity measurements and SDS-PAGE.

Amino acid transport and crystallin synthesis in the bovine lens

Bovine lenses were incubated in a defined, bicarbonate-free culture medium (EMEM) and the kinetics of amino acid uptake and protein synthesis investigated. The kinetics were interpreted in terms of a simple multi-compartment model. [14C]tyrosine was found to be totally exchangeable in the incubated lens and the rate constant for the exponential increase in activity was 0 . 0175 hr-1. The rate of influx was markedly reduced by incubating in the presence of ouabain (10(-5) M), which also caused a concomitant disturbance of the normal sodium and potassium distributions. The soluble proteins from the incubated lenses were fractionated on Sephadex G-200 and the rate of incorporation into the crystallins was shown to fall into two classes. The rate of synthesis of alpha and beta L crystallin was relatively rapid (rate constants approximately equal to 0 . 004 hr-1), while the synthesis rates of beta H and beta S/gamma were both much slower (0 . 001 hr-1). The efflux kinetics of [14C]tyrosine were determined and the rate of decrease of the free amino acid pool was identical to the rate of increase determined from an influx experiment. Hence the lenses are in a steady state with respect to free tyrosine throughout the incubation period (up to 160 hr). All classes of proteins continued to be synthesized during efflux experiments and there was no evidence for a breakdown of alpha or beta L crystallin during the time-course of these experiments. Ouabain slowed the rate of loss of tyrosine from the free amino acid pool, and this was interpreted in terms of an ouabain-induced decrease in synthesis rate rather than as a decrease in efflux rate from the lens. There was in fact a very marked decrease in the incorporation of [14C]tyrosine into the alpha and beta L crystallins on exposure to ouabain, and this decrease was apparent before any change in activity in the amino acid pool.

Persistence of crystallin messenger RNA's with reduced translation in hereditary cataracts in mice

In vitro translation experiments showed that the lens fiber cells of two hereditary cataracts in mice (Nakano and Philly) possessed a full complement of crystallin messenger RNA's, despite severely reduced synthesis of crystallin in these cells. The reduction in synthesis in the lens fiber cells correlated with the increase in Na+ and the decrease in K+, which occurs during cataractogenesis. In contrast to the fiber cells, the epithelial cells continued to synthesize crystallins in the cataractous lenses. Crystallin synthesis was stimulated in the fiber cells by raising the K+ concentration and lowering the Na+ concentration in the cultured lenses. The reduction in crystallin synthesis in the initial stages of cataractogenesis in the Nakano and Philly lenses thus appears to be due to poor utilization of crystallin messenger RNA's in the fiber cells.

Messenger RNA for cataractous lens proteins are also present on normal lens polyribosomes

We have previously described an experimental model in vivo where cataract is induced by injection of the antimitotic bleomycin in the newborn rat. The first opacity of the lens appears 12--15 days after the injection of the drug concomitantly with a group of precise biochemical modifications among the soluble crystallins. These modifications are mainly the accumulation of two additional low-molecular-weight beta crystallin subunits (beta L subunits) and of several smaller alpha-crystallin polypeptides. Messenger RNA isolated from normal and cataractous lenses was assayed for translation in a cell-free wheat germ extract. Analysis of the translation products by one-dimensional and two-dimensional gel electrophoresis indicated that the messenger RNAs coding for the two beta L subunits are also present on normal lens polyribosomes. Purification and subsequent analysis by peptide mapping of the cataractous Beta L subunits suggest that they are precursor polypeptides of the normal low-molecular-weight beta-crystallins, which are no longer processed after translation and therefore accumulate in the pathological lens cell. On the other hand, in the translation of the cataractous mRNA in vitro, only the normal alpha chains are detected. These cataractous alpha-crystallins are therefore post-translational degradation products of the normal alpha polypeptides. This phenomenon seems similar to the one observed in the senescent lens. The possible involvement of these modifications in the etiology of this experimental cataract is discussed further.

[Effect of ouabain on the lactogenic action of prolactin and on the level of mammary prolactin receptors]

Ouabain added to the culture medium of rabbit mammary gland inhibits prolactin action on the initiation of lactose and casein synthesis. The degree of inhibition is a function of the ouabain concentration in the medium. Likewise, ouabain blocks the accumulation of casein mRNA supported by prolactin. In addition, ouabain provokes a rapid disappearance of prolactin receptors. Conversely prolactin keeps its capacity to enhance the concentration of casein mRNA and the parallel casein synthesis when K+ ions are totally absent from the culture medium. These results suggest that although prolactin induces a modification of the K+/Na+ ratio in the mammary cell and ouabain prevents this effect of prolactin, the inhibitory action of ouabain on lactogenesis can be explained essentially by its effect on the hormone receptors.

Cataractogenesis: developmental inputs and constraints

The developmental period during which the lens is susceptible to one or another cataractogenic influence, the types of cataracts that result, and the spatial and temporal patterns in which these cataracts become manifest are better understood in the context of the orderly sequence of steps by which the vertebrate lens develops. Thus, events attending lens induction, formation of the lens vesicle, deposition of the lens capsule, maturation of lens fibers, elaboration of lens sutures, and developmental changes in the relationship of the lens to the optic cup have implications for cataractogenesis. We have also seen that events in juxtalenticular tissues (eg, the zonule, the temporary hyaloid-lenticular vascular, the neural retina) can contribute to the development of lenticular opacities. Hereditary cataracts, as well as congenital cataracts produced in response to untoward environmental input, can be properly understood only within a dynamic developmental context.