Gamma III-crystallin is the primary target of glycation in the bovine lens incubated under physiological conditions

LncRNA KCNQ1OT1 promotes apoptosis and oxidative stress of human lens epithelial cells through epigenetic regulation of WRN

PURPOSE

Long non-coding RNA KCNQ1OT1 is fundamental to age-related cataract (ARC), whereas the underlying mechanism is still unknown. Here, we explored the possible mechanism of KCNQ1OT1 in ARC.

METHODS

The expression of KCNQ1OT1 in ARC patients and H₂O₂-treated human lens epithelial cell line SRA01/04 was detected. Gene and protein expression were examined by quantitative real-time PCR and western blot. Cell viability and apoptosis were detected by CCK-8 assay and flow cytometry. The content of reactive oxygen species (ROS) was assessed by fluorescent probe DCFH-DA. The relationship among KCNQ1OT1, G9a, H3K9me1/2 and WRN was verified by RNA pull down and Chromatin immunoprecipitation.

RESULTS

KCNQ1OT1 was up-regulated in the anterior lens capsule tissues of ARC patients and H₂O₂-treated SRA01/04 cells. KCNQ1OT1 overexpression suppressed cell viability and facilitated apoptosis in H₂O₂-treated SRA01/04 cells. KCNQ1OT1 up-regulation enhanced the levels of ROS and malondialdehyde (MDA), and reduced the levels of superoxide dismutase (SOD) and catalase (CAT) in H₂O₂-treated SRA01/04 cells. WRN up-regulation led to a result opposite to KCNQ1OT1 overexpression. The influence of WRN up-regulation on cell viability, apoptosis and oxidative stress of SRA01/04 cells was rescued by KCNQ1OT1 overexpression. Additionally, KCNQ1OT1 interacted with G9a. Both G9a and H3K9me1/2 interacted with WRN promoter. G9a deficiency significantly enhanced WRN expression and repressed H3K9me1/2 expression in SRA01/04 cells, which was abrogated by KCNQ1OT1 up-regulation.

CONCLUSION

This study demonstrated that KCNQ1OT1 promoted apoptosis and oxidative stress of human LECs through G9a-driven epigenetic regulation of WRN. This work highlights a novel lncRNA involving key regulators of ARC.

Structural stability and solubility of glycated camel lens ζ-crystallin

The camel has several biochemical, physiological, and anatomical features to withstand the harsh desert climate. Camel eye lens contains a novel protein (ζ-crystallin) in bulk quantity. Previous reports suggest that non-enzymatic glycation of eye lens proteins plays an important role in the etiology of cataract. In this study, we have characterized the role of glucose, fructose, and methylglyoxal (MGO) in the glycation of camel lens ζ-crystallin. From the results obtained, it was found that MGO reacted rapidly, fructose reacted moderately, and glucose was the least reactive even after prolonged incubation (>100 days). Glycation with MGO and fructose led to changes in the structure of ζ-crystallin, while glucose had no remarkable effect. The surface hydrophobicity did not change and no aggregates or amyloid fibrils were observed in the glycated ζ-crystallin. Moreover, the secondary structure of glycated ζ-crystallin remained similar after glycation. Our results suggested that due to natural adaptation, the camel lens protein ζ-crystallin retained its structure and solubility even after glycation to perform the single known function of the lens proteins: to focus unscattered light on the retina.

Glycation With Fructose: The Bitter Side of Nature's Own Sweetener

Fructose is a ketohexose and sweetest among all the natural sugars. Like other reducing sugars, it reacts readily with the amino- and nucleophilic groups of proteins, nucleic acids and other biomolecules resulting in glycation reactions. The non-enzymatic glycation reactions comprise Schiff base formation, their Amadori rearrangement followed by complex and partly incompletely understood reactions culminating in the formation of Advance Glycation End products (AGEs). The AGEs are implicated in complications associated with diabetes, cardiovascular disorders, Parkinson's disease, etc. Fructose is highly reactive and forms glycation products that differ both in structure and reactivity as compared to those formed from glucose. Nearly all tissues of higher organisms utilize fructose but only a few like the ocular lens, peripheral nerves erythrocytes and testis have polyol pathway active for the synthesis of fructose. Fructose levels rarely exceed those of glucose but, in tissues that operate the polyol pathway, its concentration may rise remarkably during diabetes and related disorders. Diet contributes significantly to the body fructose levels however, availability of technologies for the large scale and inexpensive production of fructose, popularity of high fructose syrups as well as the promotion of vegetarianism have resulted in a remarkable increase in the consumption of fructose. In vivo glycation reactions by fructose, therefore, assume remarkable significance. The review, therefore, aims to highlight the uniqueness of glycation reactions with fructose and its role in some pathophysiological situations.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

Role of crystallins in diabetic complications

BACKGROUND

Crystallins are the major structural proteins of vertebrate eye lens responsible for maintaining the refractive index of the lens. However, recent studies suggest that they also have a functional significance in non-lenticular tissues. Prolonged uncontrolled diabetes results in the development of macro and microvascular complications that are the leading causes of morbidity and mortality in diabetic patients all over the world.

SCOPE OF REVIEW

Recent studies have shown that crystallins play an instrumental role in diabetes and its complications. Therefore, this review highlights the current data on the impact of chronic hyperglycemia on expression, distribution, glycation, phosphorylation, chaperone-like function and, anti-apoptotic activity of crystallins. Furthermore, we discussed the insights for developing therapeutic strategies for diabetic complications including natural agents, peptides, and pharmacological chaperones that modulate or mimic chaperone activity of α-crystallins.

MAJOR CONCLUSIONS

Upregulation of crystallins appears to be a common feature of chronic diabetes. Further, chronic hyperglycemia induces the glycation and phosphorylation of crystallins, mainly α-crystallins and thereby alters their properties. The disturbed interaction of αB-crystallin with various apoptotic mediators including Bax and caspases is also an important factor for increased cell death in diabetes. Numerous dietary agents, peptides, and chemical chaperones prevent apoptosis and the loss of chaperone activity in diabetes.

GENERAL SIGNIFICANCE

Understanding the role of crystallins will aid in developing therapeutic strategies for alleviating pathophysiological conditions such as protein aggregation, inflammation, oxidative stress and apoptosis associated with chronic complications of diabetes including cataract, retinopathy, and cardiomyopathy. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Identification of the preferentially targeted proteins by carbamylation during whole lens incubation by using radio-labelled potassium cyanate and mass spectrometry

AIM

To attempt to identify the primary targets of carbamylation in bovine lenses incubated under physiological condition.

METHODS

Fresh intact bovine lenses were incubated with [(14)C]-labelled potassium cyanate for seven days. The water-soluble proteins (WSP) of both cortex and nucleus lens were isolated by size-exclusion chromatography on a Sephacryl S-300HR column. The higher radioactive fractions were pooled and freeze-dried, and separated further by loading on an Affinity Blue column to separate some enzymes. In addition, WSP from cortex was separated directly by affinity chromatography. The most reactive fractions with higher radioactivity from [(14)C]-cyanate were further analyzed by SDS-gels and mass spectrometry.

RESULTS

The majority of protein incorporating [(14)C]-labelled potassium cyanate was in the water-soluble fractions, and much more in the cortex than in the nucleus. Chromatography results demonstrated that the major incorporated [(14)C]-carbamylated crystallins were fractions corresponding to α-crystallin, β-crystallin and ξ-crystallin in the cortex, but β-crystallin and γ-crystallin in the nucleus. The SDS gels showed that bound fractions of cortex crystallins after Affi-Gel Blue separation were abundant with 20 and 35kDa proteins. However, the bound fractions of nucleus crystallins mainly showed 20kDa proteins. Mass spectrometry analysis of these higher radioactivity fractions and a database search revealed that the proteins were originated from bovine α-crystallin A and B chains and ξ-crystallin in the cortex; βA1 and αB-crystallins with a little γB-crystallin in the nucleus respectively. Further analysis suggested the location of this carbamylation of αB-crystallin in the nucleus to be at Lys 92 and 103.

CONCLUSION

α-and ξ-crystallin from cortex can be preferentially targeted by carbamylation during whole lens incubations. Carbamylation of these crystallins at the earlier stage may result in further unfolding and misfolding of lens proteins, leading to aggregation of crystallins and eventually to cataract formation.

Identification of the primary targets of carbamylation in bovine lens proteins by mass spectrometry

PURPOSE

Carbamylation, an important post-translational modification of proteins, inevitably causes conformational changes of lens proteins. It may increase aggregation between crystallin molecules and disrupt the close packing required for transparency thus leading to cataract. The aim of this study was to isolate the primary targets of carbamylation in the lens and identify them by mass spectrometry.

MATERIALS AND METHODS

Fresh intact bovine lenses were incubated with 100 mM potassium cyanate for 7 days. The proteins in the water-soluble fractions from the normal control and the cyanate-modified lens proteins were separated by two-dimensional (2-D) gel electrophoresis with identification after silver staining. Protein spots that differed between the normal and carbamylated groups were selected for further analysis using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

RESULTS

The 2-D gel results showed that the major lens proteins were in the section of pI 5-8, with relative molecular masses of 20-35 kDa, and changes in the carbamylated fraction like strings of beads indicating modification. The mass spectrometry analysis and a database search identified carbamylated proteins originating from alphaA-crystallin, betaB2- and gammaS-(betaS)-crystallins.

CONCLUSIONS

These crystallins may be vulnerable proteins targeted by carbamylation. The accumulated aggregation and loss of chaperone activity may contribute to cataract formation.

Glucose-derived Amadori compounds of glutathione

Under the chromatographic conditions used in these studies we observed time- and concentration-dependent formation of N-1-Deoxy-fructos-1-yl glutathione as the major glycation product formed in the mixtures of GSH with glucose. N-1-Deoxy-fructos-1-yl glutathione had a characteristic positively charged ion with m/z=470 Th in its LC-MS spectra. Mixtures of glutathione disulfide and glucose generated two compounds: N-1-Deoxy-fructos-1-yl GSSG (m/z=775 Th) as major adduct and bis di-N, N'-1-Deoxy-fructos-1-yl GSSG (m/z=937 Th) as the minor one. All three compounds showed a resonance signal at 55.2 ppm in the 13C-NMR spectra as C1 methylene group of deoxyfructosyl, which represents direct evidence that they are Amadori compounds. All three compounds purified from GSSG/Glc or GSH/Glc mixtures also showed LC-MS/MS fragmentation patterns identical to those of the synthetically synthesized N-1-Deoxy-fructos-1-yl glutathione, N-1-Deoxy-fructos-1-yl GSSG and bis di-N, N'-1-Deoxy-fructos-1-yl GSSG. N-1-Deoxy-fructos-1-yl glutathione was shown to be a poor substrate for glutathione peroxidase (6.7% of the enzyme's original specific activity) and glutathione-S-transferase (25.7% of the original enzyme's specific activity). Glutathione reductase failed to recycle the disulfide bond within the structure of di-substituted bis di-N, N'-1-Deoxy-fructos-1-yl GSSG. It showed only 1% of the original enzyme's specific activity, but retained its ability to reduce the disulfide bond within the structure of N-1-Deoxy-fructos-1-yl GSSG by 57% of its original specific activity. Since the GSH concentration in diabetic lens is significantly decreased and the glucose concentration can increase 10-fold and higher, the formation of Amadori products of the different forms of glutathione with this monosaccharide may be favored under these conditions and could contribute to a lowering of glutathione levels and an increase of oxidative stress observed in diabetic lens.