Biosynthetic capacity of the human lens upon aging

Gene profiles and mutations in the development of cataracts in the ICR rat model of hereditary cataracts

Cataracts are opacifications of the lens that cause loss of visual acuity and ultimately of eyesight. Age-related cataract develops in most elderly people, but the mechanisms of cataract onset are incompletely understood. The Ihara Cataract Rat (ICR) is an animal model of hereditary cataracts showing cortical opacity that commonly develops prematurely. We identified putative mechanisms of cataract onset in the ICR rat model by measuring gene expression changes before and after cortical cataract development and conducting point mutation analysis. Genes differentially expressed between 4-week-old animals without cortical cataracts and 8-10-week-old animals with cortical cataracts were selected from microarray analysis. Three connections were identified by STRING analysis: (i) Epithelial-Mesenchymal Transition (EMT), including Col1a2, and Pik3r1. (ii) Lens homeostasis, including Aqp5, and Cpm. (iii) Lipid metabolism, including Scd1, Srebf1, and Pnpla3. Subsequently, mutation points were selected by comparing ICR rats with 12 different rats that do not develop cataracts. The apolipoprotein Apoc3 was mutated in ICR rats. Analyses of gene expression changes and point and mutations suggested that abnormalities in EMT or lipid metabolism could contribute to cataract development in ICR rats.

Protecting the Eye Lens from Oxidative Stress through Oxygen Regulation

Molecular oxygen is a primary oxidant that is involved in the formation of active oxygen species and in the oxidation of lipids and proteins. Thus, controlling oxygen partial pressure (concentration) in the human organism, tissues, and organs can be the first step in protecting them against oxidative stress. However, it is not an easy task because oxygen is necessary for ATP synthesis by mitochondria and in many biochemical reactions taking place in all cells in the human body. Moreover, the blood circulatory system delivers oxygen to all parts of the body. The eye lens seems to be the only organ that is protected from the oxidative stress through the regulation of oxygen partial pressure. The basic mechanism that developed during evolution to protect the eye lens against oxidative damage is based on the maintenance of a very low concentration of oxygen within the lens. This antioxidant mechanism is supported by the resistance of both the lipid components of the lens membrane and cytosolic proteins to oxidation. Any disturbance, continuous or acute, in the working of this mechanism increases the oxygen concentration, in effect causing cataract development. Here, we describe the biophysical basis of the mechanism and its correlation with lens transparency.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Three-Dimensional Human Cell Culture Models to Study the Pathophysiology of the Anterior Eye

In recent decades, the establishment of complex three-dimensional (3D) models of tissues has allowed researchers to perform high-quality studies and to not only advance knowledge of the physiology of these tissues but also mimic pathological conditions to test novel therapeutic strategies. The main advantage of 3D models is that they recapitulate the spatial architecture of tissues and thereby provide more physiologically relevant information. The eye is an extremely complex organ that comprises a large variety of highly heterogeneous tissues that are divided into two asymmetrical portions: the anterior and posterior segments. The anterior segment consists of the cornea, conjunctiva, iris, ciliary body, sclera, aqueous humor, and the lens. Different diseases in these tissues can have devastating effects. To study these pathologies and develop new treatments, the use of cell culture models is instrumental, and the better the model, the more relevant the results. Thus, the development of sophisticated 3D models of ocular tissues is a significant challenge with enormous potential. In this review, we present a comprehensive overview of the latest advances in the development of 3D in vitro models of the anterior segment of the eye, with a special focus on those that use human primary cells.

Synchrotron-based FTIR microspectroscopy of protein aggregation and lipids peroxidation changes in human cataractous lens epithelial cells

Cataract is the leading cause of blindness worldwide but the mechanisms involved in the process of cataractogenesis are not yet fully understood. Two most prevalent types of age-related cataracts are nuclear (N) and cortical (C) cataracts. A common environmental factor in most age-related cataracts is believed to be oxidative stress. The lens epithelium, the first physical and biological barrier in the lens, is build from lens epithelial cells (LECs). LECs are important for the maintenance of lens transparency as they control energy production, antioxidative mechanisms and biochemical transport for the whole lens. The purpose of this study is to characterize compounds in LECs originated from N and C cataracts, by using the synchrotron radiation-based Fourier Transform Infrared (SR-FTIR) microspectroscopy, in order to understand the functional importance of their different bio-macromolecules in cataractogenesis. We used the SR-FTIR microspectroscopy setup installed on the beamline MIRAS at the Spanish synchrotron light source ALBA, where measurements were set to achieve single cell resolution, with high spectral stability and high photon flux. The results showed that protein aggregation in form of fibrils was notably pronounced in LECs of N cataracts, while oxidative stress and the lipids peroxidation were more pronounced in LECs of C cataracts.

Why Is Very High Cholesterol Content Beneficial for the Eye Lens but Negative for Other Organs?

The plasma membranes of the human lens fiber cell are overloaded with cholesterol that not only saturates the phospholipid bilayer of these membranes but also leads to the formation of pure cholesterol bilayer domains. Cholesterol level increases with age, and for older persons, it exceeds the cholesterol solubility threshold, leading to the formation of cholesterol crystals. All these changes occur in the normal lens without too much compromise to lens transparency. If the cholesterol content in the cell membranes of other organs increases to extent where cholesterol crystals forma, a pathological condition begins. In arterial cells, minute cholesterol crystals activate inflammasomes, induce inflammation, and cause atherosclerosis development. In this review, we will indicate possible factors that distinguish between beneficial and negative cholesterol action, limiting cholesterol actions to those performed through cholesterol in cell membranes and by cholesterol crystals.

Whales, lifespan, phospholipids, and cataracts

This study addresses the question: why do rats get cataracts at 2 years, dogs at 8 years, and whales do not develop cataracts for 200 years? Whale lens lipid phase transitions were compared with the phase transitions of other species that were recalculated. The major phospholipids of the whale lens were sphingolipids, mostly dihydrosphingomyelins with an average molar cholesterol/phospholipid ratio of 10. There was a linear correlation between the percentage of lens sphingolipid and lens lipid hydrocarbon chain order until about 60% sphingolipid. The percentage of lens sphingolipid correlated with the lens lipid phase transition temperature. The lifespan of the bowhead whale was the longest of the species measured and the percentage of whale lens sphingolipid fit well in the correlation between the percentage of lens sphingolipid and lifespan for many species. In conclusion, bowhead whale lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these lenses to stay clear relatively longer than many other species. The strong correlation between sphingolipid and lifespan may form a basis for future studies, which are needed because correlations do not infer cause. One could hope that if human lenses could be made to have a lipid composition similar to whales, like the bowhead, humans would not develop age-related cataracts for over 100 years.

Lipids and the ocular lens

The unusually high levels of saturation and thus order contribute to the uniqueness of human lens membranes. In addition, and unlike in most biomembranes, most of the lens lipids are associated with proteins, thus reducing their mobility. The major phospholipid of the human lens is dihydrosphingomyelin. Found in significant quantities only in primate lenses, particularly human ones, this lipid is so extremely stable that it was reported to be the only lipid remaining in a frozen mammoth 40,000 years after its death. Unusually high levels of cholesterol add peculiarity to the composition of lens membranes. Beyond the lateral segregation of lipids into dynamic domains known as rafts, the high abundance of cholesterol in the human lens leads to the formation of patches of pure cholesterol. Changes in human lens lipid composition with age and disease as well as differences among species are greater than those observed for any other biomembrane. The relationships among lens membrane composition, structure, and lipid conformation reviewed in this article are unique to the mammalian lens and offer exciting insights into lens membrane function. This review focuses on findings reported over the last two decades that demonstrate the uniqueness of mammalian lens membranes regarding their morphology and composition. Because the membranes of human lenses do undergo the most dramatic changes with age and cataractogenesis, the final sections of this review address our current knowledge of the unusual composition and organization of adult human lens membranes with and without opacification. Finally, the questions that still remain to be answered are presented.

A current practice for predicting ocular toxicity of systemically delivered drugs

The ability to predict ocular side effects of systemically delivered drugs is an important issue for pharmaceutical companies. Although animal models involving standard clinical ophthalmic examinations and postmortem microscopic examinations of eyes are still used to identify ocular issues, these methods are being supplemented with additional in silico, in vitro, and in vivo techniques to identify potential safety issues and assess risk. The addition of these tests to a development plan for a potential new drug provides the opportunity to save time and money by detecting ocular issues earlier in the program. This review summarizes a current practice for minimizing the potential for systemically administered, new medicines to cause adverse effects in the eye.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

Discordant expression of the sterol pathway in lens underlies simvastatin-induced cataracts in Chbb: Thom rats

Simvastatin rapidly induced cataracts in young Chbb:Thom (CT) but not Sprague Dawley (SD) or Hilltop Wistar (HW) rats. Oral treatment for 14 but not 7 days committed CT rat lenses to cataract formation. The cholesterol to phospholipid molar ratio in lenses of treated CT rats was unchanged. Differences between strains in serum and ocular humor levels of simvastatin acid poorly correlated with susceptibility to cataracts. No significant differences were found between rat strains in the capacity of simvastatin acid to inhibit lens-basal sterol synthesis. Prolonged treatment with simvastatin comparably elevated HMG-CoA reductase protein and enzyme activity in lenses of both cataract resistant and sensitive strains. However, in contrast to SD and HW rats, where sterol synthesis was markedly increased, sterol synthesis in CT rat lenses remained at baseline. Discordant expression of sterol synthesis in CT rats may be due to inadequate upregulation of lens HMG-CoA synthase. HMG-CoA synthase protein levels, and to a much lesser extent mRNA levels, increased in lens cortex of SD but not CT rats. Because upregulation of the sterol pathway may result in increased formation of isoprene-derived anti-inflammatory substances, failure to upregulate the pathway in CT rat lenses may reflect an attenuated compensatory response to injury that resulted in cataracts.

Is there a glucocorticoid receptor in the bovine lens?

Prolonged glucocorticoid therapy is a risk factor for cataract development. The mechanism remains unknown. If cataract results from the direct effect of steroids on lens function, a glucocorticoid receptor is required. In order to determine whether such a receptor was present in the bovine lens, metabolic and steroid binding experiments were undertaken. Cultured bovine lens epithelial cells were exposed to 10(- 4)and 10(-8) M dexamethasone or prednisolone and the uptake and incorporation of(14)C leucine,(14)C glucose and(3)H thymidine, examined. Neither glucocorticoid affected cell protein synthesis or glucose uptake. Both dexamethasone concentrations and the lower concentration of prednisolone had no effect on thymidine uptake or incorporation, however, the 10(-4) M prednisolone exposure reduced these by 15 +/- 5%. This regulation is thought to be due to membrane fluidity changes and not the action of the glucocorticoid receptor. As the glucocorticoid receptor is very heat labile in vitro, the effects of increasing temperature on dexamethasone binding by proteins from lens epithelium, lens nucleus and liver were examined. At 0 degree C, lens epithelial extract bound nine-fold more dexamethasone than liver extract. After exposure to 37 degrees C, liver binding decreased by 66% whereas that for lens epithelium increased by 18%. For both lens extracts, steroid binding increased with temperature up to 50 degrees C. Scatchard analysis of the steroid binding kinetics showed there to be no high affinity sites in lens epithelial extract, with the binding best described as a non-specific partitioning event. Western blotting with a specific glucocorticoid receptor antibody revealed protein bands of approximately 94 and 79 kDa in liver, which is known to contain significant levels of receptor. No immunoreactivity was observed for lens epithelial extract. Therefore, within the limits of detection, these results suggest the bovine lens does not contain a glucocorticoid receptor. This raises questions about the validity of receptor-mediated mechanisms proposed for cataract development.

Age-related changes in human lens crystallins identified by two-dimensional electrophoresis and mass spectrometry

The purpose of this study was to identify the major protein components in adult human lenses and to analyse the specific age-related changes in these proteins using two-dimensional electrophoresis, Edman sequencing, and in conjunction with the data in the accompanying manuscript, mass spectrometry. The majority of changes in the two-dimensional electrophoretic pattern of lens proteins occurred prior to 17 years of age, and included a decrease in proteins migrating to the original positions of beta B1, beta B3, beta A3, gamma C and gamma D, and the appearance of many new species with apparent molecular weights on two-dimensional electrophoretic gels similar to beta B2 and gamma S, but having more acidic pIs. These proteins were identified as deamidated forms of beta B1 and beta A3/A1 missing portions of their N-terminal extensions. With the exception of alpha B, deamidation was detected in all crystallin species. These data indicated that a major fraction of the water-soluble protein of the adult human lens is composed of truncated beta B1 and beta A3/A1 crystallins, and that nearly all human crystallins, including the, beta-crystallins, are susceptible to deamidation. The results also provided the most detailed map to date of the identities of protein species on two-dimensional electrophoresis gels of adult human lenses.

Posttranscriptional regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in lens epithelial cells by mevalonate-derived nonsterols

The ocular lens must continuously synthesize the cholesterol required to support membrane formation for its life-long growth. The roles of transcriptional and posttranscriptional mechanisms in controlling 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) protein levels in cultured lens epithelial cells were examined by measuring the effect of restricting exogenous cholesterol, endogenous cholesterol synthesis and mevalonate derived nonsterols upon HMGR protein and mRNA levels and upon the synthesis and degradation of HMGR protein. Sterols were restricted by culturing in lipoprotein deficient media and blocking 2,3-oxidosqualene cyclase with U18666A. Mevalonate derived nonsterols were additionally restricted by inhibition of HMGR activity with lovastatin. A 4-fold increase in HMGR protein levels due to restricting sterols with U18666A could be explained by comparably increased mRNA levels and enzyme protein synthesis. The very rapid turnover of HMGR protein (T(1/2) approximately 45 min) was unaffected. The additional restriction of mevalonate derived nonsterols increased HMGR protein levels to about 400-fold. A 10-fold slowing in the rate of enzyme degradation coupled with at least a 5-fold increase in mRNA levels likely accounted for this accumulated protein mass. The capacity of the nonsterol regulators to promote enzyme degradation appeared independent of sterols, since mevalonate restored rapid degradation of HMGR protein when 2,3-oxidosqualene cyclase activity was simultaneously blocked. Thus, in cultured lens epithelial cells, sterols appear to exert a modest influence on HMGR protein levels solely by suppressing transcription; whereas, mevalonate derived nonsterols exert major influence mainly by accelerating enzyme protein degradation. We speculate that nonsterol isoprenes might be important for preventing overexpression of cholesterol biosynthesis in the intact lens.

Sequence analysis of betaA3, betaB3, and betaA4 crystallins completes the identification of the major proteins in young human lens

A combination of Edman sequence analysis and mass spectrometry identified the major proteins of the young human lens as alphaA, alphaB, betaA1, betaA3, betaA4, betaB1, betaB2, betaB3, gammaS, gammaC, and gammaD-crystallins and mapped their positions on two-dimensional electrophoretic gels. The primary structures of human betaA1, betaA3, betaA4, and betaB3-crystallin subunits were predicted by determining cDNA sequences. Mass spectrometric analyses of each intact protein as well as the peptides from trypsin-digested proteins confirmed the predicted amino acid sequences and detected a partially degraded form of betaA3/A1 missing either 22 or 4 amino acid residues from its N-terminal extension. These studies were a prerequisite for future studies to determine how human lens proteins are altered during aging and cataract formation.

Calcium mobilisation modulates growth of lens cells

The modulating effect of calcium cell signalling agonists on tissue growth was studied in a rabbit lens cell line (NN1003A). Calcium mobilisation was measured after Fura-2 incorporation and growth assayed either by direct Coulter counting or [3H]-thymidine incorporation. Transient increases in cytoplasmic calcium were elicited by rabbit serum, histamine, ATP and PDGF. Thapsigargin induced a prolonged increase and all of the above agonists failed to elicit a response after thapsigargin. Rabbit serum and PDGF both increased cell growth in a concentration-dependent manner. While histamine and ATP had little effect in serum-free medium, they reduced serum-stimulated growth. Acetylcholine and FGF did not produce a marked rise in cytoplasmic calcium and neither did they modulate growth. Both thapsigargin and caffeine greatly inhibited growth. These findings indicate that, in lens cells, agonists that mobilise calcium, whether by acting through G-protein or tyrosine kinase receptors, also modulate lens cell growth. Agents such as thapsigargin and caffeine that inactivate the same calcium store also inhibit growth.

Cholesterol and cataracts

Inherited defects in enzymes of cholesterol metabolism and use of drugs which inhibit lens cholesterol biosynthesis can be associated with cataracts in animals and man. The basis of this relationship apparently lies in the need of the lens to satisfy its sustained requirement for cholesterol by on-site synthesis, and impairing this synthesis can lead to alteration of lens membrane structure. Lens membrane contains the highest cholesterol content of any known membrane. The Smith-Lemli-Opitz syndrome, mevalonic aciduria, and cerebrotendinous xanthomatosis all involve mutations in enzymes of cholesterol metabolism, and affected patients can develop cataracts. Two established models of rodent cataracts are based on treatment with inhibitors of cholesterol biosynthesis. The long-term ocular safety of the very widely used vastatin class of hypocholesterolemic drugs is controversial. Some vastatins are potent inhibitors of cholesterol biosynthesis by animal lenses, can block cholesterol accumulation by these lenses and can produce cataracts in dogs. Whether these drugs inhibit cholesterol biosynthesis in human lenses at therapeutic doses is unknown. Results of clinical trials of 1-5 years duration in older patient populations indicate high ocular safety. However, considering the slow life-long growth of the lens and its continuing need for cholesterol, longterm safety of the vastatins should perhaps be viewed in units of 10 or 20 years, particularly with younger patients.

Spectral characterization of lipid peroxidation in rabbit lens membranes induced by hydrogen peroxide in the presence of Fe2+/Fe3+ cations: a site-specific catalyzed oxidation

The role of free-radical-induced lipid peroxidation (LPO) in relation to lens opacity is investigated using Fourier transform infrared spectroscopy. Phospholipids extracted from nuclear and cortical regions of the rabbit lens membranes are subjected to oxidative-damage induced by hydrogen peroxide and Fe2+/Fe3+ cations. Vibrational data suggest a homolytic decomposition of the unsaturated membrane hydrocarbon chains at cis-double bonds, as well as structural modifications at the carbonyl and phosphate-oxygen sites of the fiber cell membranes upon metal oxidation. This is also evident from a substantial induction of the carbonyl groups and a significant dephosphorylation of the phosphate groups in lens phospholipids. These covalent modifications and/or alterations of the carbonyl and phosphate groups, and specificity of certain vibrational modes only to iron oxidation, may serve as a diagnostic probe of the metal-catalyzed LPO in lens membranes. Despite covalent modifications of the hydrophilic part of the lens membranes, hydrocarbon chain region remains largely intact at physiological concentrations of hydrogen peroxide. However, at elevated concentrations of hydrogen peroxide, a substantial breakdown of the acyl chains occurs. Striking similarities observed between the spectral features of the oxidized rabbit lens phospholipids and those of the cataractous human lenses suggest that the mechanism and pathways of lipid oxidation in model animal membranes and in human lenses are similar. Differences in the nuclear or cortical regions are also evident upon metal oxidation. Nuclear lipids experience increased effects of the metal oxidation compared to cortical lipids. Both the nuclear or the cortical lipids indicate effective penetration of the bilayer water creating segregated membrane domains, possibly through breakdown of headgroup-specific lipid-water interactions. This could effectively alter the lens membrane permeability and fluidity, rendering it susceptible to a host of toxic oxidants present in the eye. These findings also demonstrate that LPO can lead to acyl chain degradation that may effectively derange the lens membrane function, which could be a contributing factor in cataractogenesis.

Different effects of the hypolipidemic drugs pravastatin and lovastatin on the cholesterol biosynthesis of the human ocular lens in organ culture and on the cholesterol content of the rat lens in vivo

This study aimed to investigate the influence of the hypocholesterolemic drugs pravastatin and lovastatin on the cholesterol biosynthesis in the ocular lens. Two model systems were used: a human lens organ culture system and an in vivo rat lens system. For measurements of cholesterol and fatty acid synthesis rates, human lenses were incubated for 20 h in the presence of [14C]acetate. Pravastatin and lovastatin were added 1 h prior to the addition of the radioactive label. In order to avoid the influence of differences relating to individual donors, one lens from each donor was incubated without drug (control) and the other lens was incubated in the presence of the drug. Statistical analysis showed that the fatty acid synthesis rate was not influenced by the drug. For each lens pair the percentage inhibition of the cholesterol synthesis caused by the drug was calculated. Using various concentrations of the drugs, a dose-response curve was composed for the inhibition of the cholesterol synthesis. The experiments showed that in the human lens organ culture system, lovastatin was 100-fold more potent than pravastatin in inhibiting the cholesterol biosynthesis. To study the in vivo influence of vastatins on the cholesterol content of the developing lens, Wistar rats were weaned at day 21 of age and subsequently the pups were fed a control diet or drug-containing diet (10, 50 or 100 mg lovastatin/kg chow) for a 3-week period. At the end of diet intervention, doses of 50 or 100 mg lovastatin/kg chow had caused a reduction of about 20% of the lenticular cholesterol content compared with controls. No effect on the lens cholesterol content by pravastatin was observed. Both human ex vivo and rat in vivo experiments show that lovastatin much more strongly inhibits the lenticular cholesterol synthesis than does pravastatin.

Two-dimensional gel electrophoretic analysis of human lens proteins

Human lens proteins from clear lenses were separated and identified using two-dimensional polyacrylamide electrophoresis. Isoelectric focusing, both equilibrium and non-equilibrium, was performed in the first dimension and SDS electrophoresis in the second dimension. Proteins were identified by Western blotting and sequencing techniques and by comparison with patterns obtained with purified crystallin fractions. Analyses were performed on total urea soluble proteins of lenses varying in age from fetal to 73 yr. Several hundred protein spots representing crystallins, cytoskeletal proteins and enzymes were resolved in the fetal lens. In the older lenses there was a dramatic increase in the number of protein species in the molecular weight range of the crystallins and a reduced number of discrete protein species visible at molecular weights greater than 50,000. Conversely, a number of proteins below approximately 15 kDa were visible even in the fetal lens. The number and amount of polypeptides in this molecular weight range were increased in the older lenses. Many of these low molecular weight species could be assigned to either the alpha-, beta- or gamma-crystallin fractions. An age dependent increase in the number of acidic species of both crystallins and other proteins, such as, glyceraldehyde 3-phosphate dehydrogenase was observed as well as the loss or mobility change of gamma-crystallin. Two-dimensional gel electrophoresis provides a sensitive and practical technique for characterizing all of the proteins of the human lens.