The importance of parameter choice in modelling dynamics of the eye lens

The lens provides refractive power to the eye and is capable of altering ocular focus in response to visual demand. This capacity diminishes with age. Current biomedical technologies, which seek to design an implant lens capable of replicating the function of the biological lens, are unable as yet to provide such an implant with the requisite optical quality or ability to change the focussing power of the eye. This is because the mechanism of altering focus, termed accommodation, is not fully understood and seemingly conflicting theories require experimental support which is difficult to obtain from the living eye. This investigation presents finite element models of the eye lens based on data from human lenses aged 16 and 35 years that consider the influence of various modelling parameters, including material properties, a wide range of angles of force application and capsular thickness. Results from axisymmetric models show that the anterior and posterior zonules may have a greater impact on shape change than the equatorial zonule and that choice of capsular thickness values can influence the results from modelled simulations

Different experimental approaches in modelling cataractogenesis: An overview of selenite-induced nuclear cataract in rats

Cataract, the opacification of eye lens, is the leading cause of blindness worldwide. At present, the only remedy is surgical removal of the cataractous lens and substitution with a lens made of synthetic polymers. However, besides significant costs of operation and possible complications, an artificial lens just does not have the overall optical qualities of a normal one. Hence it remains a significant public health problem, and biochemical solutions or pharmacological interventions that will maintain the transparency of the lens are highly required. Naturally, there is a persistent demand for suitable biological models. The ocular lens would appear to be an ideal organ for maintaining culture conditions because of lacking blood vessels and nerves. The lens in vivo obtains its nutrients and eliminates waste products via diffusion with the surrounding fluids. Lens opacification observed in vivo can be mimicked in vitro by addition of the cataractogenic agent sodium selenite (Na2SeO3) to the culture medium. Moreover, since an overdose of sodium selenite induces also cataract in young rats, it became an extremely rapid and convenient model of nuclear cataract in vivo. The main focus of this review will be on selenium (Se) and its salt sodium selenite, their toxicological characteristics and safety data in relevance of modelling cataractogenesis, either under in vivo or in vitro conditions. The studies revealing the mechanisms of lens opacification induced by selenite are highlighted, the representatives from screening for potential anti-cataract agents are listed

Acid-induced dissociation of alpha A- and alpha B-crystallin homopolymers.

Homopolymers were constructed from the alpha A and alpha B polypeptides isolated from the lens protein alpha-crystallin. As the pH is lowered from 7.0 to 3.4, these homopolymers dissociate to smaller species with molecular masses ranging from 80 to 250 kDa for the alpha A and around 140 kDa for the alpha B dissociation products. The pKa for this dissociation was 3.8 +/- 0.2 for alpha A and 4.1 +/- 0.1 for alpha B homopolymers. Further decreases in pH, to 2.5, resulted in the presence of only denatured alpha B polypeptides, whereas the alpha A dissociation products remained intact. Fractionation of the acid dissociation products from the alpha A homopolymer at pH 2.5 yielded stable species with molecular masses of 220 +/- 30, 160 +/- 20, and 90 +/- 10 kDa. The majority of the population at acid pH consisted of the 160 kDa species. Conformational analysis of these species revealed that most of the secondary structure of the original alpha A homopolymer was retained but that the tertiary structure was perturbed. Fluorescence quenching and energy transfer measurements suggested that the molecule had undergone acid expansion, with the greatest perturbation observed in the smallest particles. The results from this work suggest that alpha A homopolymers are heterogeneous populations of aggregates of a "monomeric" molecule with a molecular mass of 160 kDa. This "monomeric" molecule may be formed from the association of two tetrameric units

Structure/function studies of dogfish alpha-crystallin, comparison with bovine alpha-crystallin

Purpose: α-Crystallin is the major protein of the mammalian lens where it contributes to the refractive properties needed for vision and possibly to the stability of the tissue. The aim of this study was to determine whether the properties of α-crystallin have changed during the course of evolution. Methods: Dogfish α-crystallin, which appeared over 420 million years ago, has been contrasted with bovine α-crystallin, which emerged around 160 million years later, by comparing their sizes, the microenvironments of their cysteine and tryptophan residues, their chaperone-like activities and the flexibility of their COOH-terminal extensions. Results: Dogfish α-crystallin consists of αA- and αB-polypeptides, in a 1:5 ratio, and has a molecular mass of around 400 kDa. By contrast, the bovine protein is around 600-800 kDa in mass and has a 3:1 subunit ratio. Cysteine residues in the proteins were equally accessible to reaction with 5,5'-dithiobis-(2-nitrobenzoic acid). Quenching of fluorescence with acrylamide indicated tryptophan residues in the two proteins were in similar environments. The chaperone activity of dogfish α-crystallin was comparable to that of bovine α-crystallin in preventing the heat-induced precipitation of βL-crystallin but the dogfish protein was three times more effective at preventing insulin precipitation after reduction at 37 ˚C. 1H nuclear magnetic resonance spectroscopic studies showed that the last 17 amino acids of the dogfish αB polypeptide (V162-K178) have great conformational flexibility, are highly exposed to solvent and adopt little ordered conformation. This is comparable to, but slightly longer in length, than the COOH-terminal extension observed in mammalian α-crystallins. Conclusions: The structure and properties of α-crystallin have changed relatively little during the evolutionary period from the emergence of sharks and mammals.A. Ghahghaei, A. Rekas, J.A. Carver, R.C. Augustey

Methylglyoxal induces endoplasmic reticulum stress and DNA demethylation in the Keap1 promoter of human lens epithelial cells and age-related cataracts

Age-related cataracts are a leading cause of blindness. Previously, we have demonstrated the association of the unfolded protein response with various cataractogenic stressors. However, DNA methylation alterations leading to suppression of lenticular antioxidant protection remains unclear. Here, we report the methylglyoxal-mediated sequential events responsible for Keap1 promoter DNA demethylation in human lens epithelial cells, because Keap1 is a negative regulatory protein that regulates the Nrf2 antioxidant protein. Methylglyoxal induces endoplasmic reticulum stress and activates the unfolded protein response leading to overproduction of reactive oxygen species before human lens epithelial cell death. Methylglyoxal also suppresses Nrf2 and DNA methyltransferases but activates the DNA demethylation pathway enzyme TET1. Bisulfite genomic DNA sequencing confirms the methylglyoxal-mediated Keap1 promoter DNA demethylation leading to overexpression of Keap1 mRNA and protein. Similarly, bisulfite genomic DNA sequencing shows that human clear lenses (n = 15) slowly lose 5-methylcytosine in the Keap1 promoter throughout life, at a rate of 1% per year. By contrast, diabetic cataractous lenses (n = 21) lose an average of 90% of the 5-methylcytosine regardless of age. Overexpressed Keap1 protein is responsible for decreasing Nrf2 by proteasomal degradation, thereby suppressing Nrf2-dependent stress protection. This study demonstrates for the first time the associations of unfolded protein response activation, Nrf2-dependent antioxidant system failure, and loss of Keap1 promoter methylation because of altered active and passive DNA demethylation pathway enzymes in human lens epithelial cells by methylglyoxal. As an outcome, the cellular redox balance is altered toward lens oxidation and cataract formation

Efficient age determination: how freezing affects eye lens weight of the small rodent species Arvicola terrestris

Age determination of animals by measuring the weight of their eye lenses is a widely used method in wildlife biology. In general, it is recommended to prepare lenses immediately after trapping to avoid errors in the age estimation due to decomposition of lens tissue. However, in many field studies, large numbers of animals need to be trapped over long periods of time in huge areas and by many different field workers. Therefore, the immediate preparation of eye lenses imposes a considerable logistic constraint that could be avoided by prior freezing of trapped animals. To assess the impact of freezing, weights of lens of frozen and unfrozen eyes of 114 Arvicola terrestris were compared pair wise. The frozen lenses weighed at average 3.3% (95% CI: 2.4–4.1%) more than the unfrozen ones from the same animals. Freezing time, weight of lenses and mean temperature of the trapping day as an indicator of decomposition speed did not affect the freezing-induced weight increase. Age estimates based on weights of unfrozen lenses varied between 24 and 445 days. Estimates based on frozen lenses were systematically higher. Applying a constant correction factor of 1.033−1 for the weight of frozen lenses corrects this overestimation of age. We conclude that age determination with frozen lenses of small rodents can yield valid age estimates if a correction factor for freezing is applied. Thus, age determination can be organised much more efficiently in field studies, which is highly advantageous for many ecological, agricultural and epidemiological research projects