Decreased chaperone activity of alpha-crystallins in naphthalene-induced cataract possibly results from C-terminal truncation

Expression of αA-crystallin (CRYAA) in vivo and in vitro models of age-related cataract and the effect of its silencing on HLEB3 cells

AIM

To investigate the expression of αA-crystallin (CRYAA) in age-related cataract (ARC) models and its role in lens epithelial cells (LECs).

METHODS

We used Flow cytometry to detect the apoptosis and cell cycle in HLEB3 cells and Real-time fluorescence quantitative polymerase chain reaction to detect the expression of CRYAA mRNA in HLEB3 and in rabbit lens. The expression of CRYAA in HLEB3 cells and rabbit lenses as well as the proteins related to apoptosis and autophagy in transfected cells were detected by western blotting. The lens structure in rabbits was investigated using hematoxylin-eosin staining. Protein thermostability assay was performed to detect the thermal stability of rabbit lens proteins. CCK- 8 assay was used to detect the viability of transfected cells, and the transfection was recorded by fluorescence photography.

RESULTS

Hydrogen peroxide can promote apoptosis and arrest the cell cycle in HLEB3 cells, and naphthalene can cause cataract formation and damage the structure of the lens in rabbits. Both ARC models can reduce the expression of CRYAA. The expression of CRYAA silencing increased apoptosis and autophagy in HLEB3 cells.

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.

Expanding the phenotype of CRYAA nucleotide variants to a complex presentation of anterior segment dysgenesis

Background

Mutations in CRYAA, which encodes the α-crystallin protein, are associated with a spectrum of congenital cataract–microcornea syndromes.

Results

In this study, we performed clinical examination and subsequent genetic analysis in two unrelated sporadic cases of different geographical origins presenting with a complex phenotype of ocular malformation. Both cases manifested bilateral microphthalmia and severe anterior segment dysgenesis, primarily characterized by congenital aphakia, microcornea, and iris hypoplasia/aniridia. NGS-based analysis revealed two novel single nucleotide variants occurring de novo and affecting the translation termination codon of the CRYAA gene, c.520T > C and c.521A > C. Both variants are predicted to elongate the C-terminal protein domain by one-third of the original length.

Conclusions

Our report not only expands the mutational spectrum of CRYAA but also identifies the genetic cause of the unusual ocular phenotype described in this report.

Proteomic analysis of protein homeostasis and aggregation

Protein homeostasis (proteostasis) refers to the ability of cells to preserve the correct balance between protein synthesis, folding and degradation. Proteostasis is essential for optimal cell growth and survival under stressful conditions. Various extracellular and intracellular stresses including heat shock, oxidative stress, proteasome malfunction, mutations and aging-related modifications can result in disturbed proteostasis manifested by enhanced misfolding and aggregation of proteins. To limit protein misfolding and aggregation cells have evolved various strategies including molecular chaperones, proteasome system and autophagy. Molecular chaperones assist folding of proteins, protect them from denaturation and facilitate renaturation of the misfolded polypeptides, whereas proteasomes and autophagosomes remove the irreversibly damaged proteins. The impairment of proteostasis results in protein aggregation that is a major pathological hallmark of numerous age-related disorders, such as cataract, Alzheimer's, Parkinson's, Huntington's, and prion diseases. To discover protein markers and speed up diagnosis of neurodegenerative diseases accompanied by protein aggregation, proteomic tools have increasingly been used in recent years. Systematic and exhaustive analysis of the changes that occur in the proteomes of affected tissues and biofluids in humans or in model organisms is one of the most promising approaches to reveal mechanisms underlying protein aggregation diseases, improve their diagnosis and develop therapeutic strategies. Significance: In this review we outline the elements responsible for maintaining cellular proteostasis and present the overview of proteomic studies focused on protein-aggregation diseases. These studies provide insights into the mechanisms responsible for age-related disorders and reveal new potential biomarkers for Alzheimer's, Parkinson's, Huntigton's and prion diseases.

Association of alpha A-crystallin polymorphisms with susceptibility to nuclear age-related cataract in a Han Chinese population

Background

Alpha A-crystallin (CRYAA) is considered critical for the maintenance of lens transparency and is related to the pathogenesis of age-related cataracts (ARCs), especially the nuclear subtype. As the 5′ untranslated region (5′ UTR) modulates gene expression, the purpose of current study was to investigate whether single nucleotide polymorphisms (SNPs) in the 5′ UTR of CRYAA were associated with susceptibility to ARC in a Han Chinese population and to clarify the mechanism of this association.

Methods

SNPs in the 5′ UTR (−1 to −1000) of CRYAA were identified in 243 nuclear ARC patients and 263 controls using polymerase chain reaction and DNA sequencing. Allele and genotype frequencies were calculated and compared between two groups. Haploview 4.2 was used to calculate the linkage disequilibrium index, and the SHEsis analysis platform was used to infer haplotype construction. A dual-luciferase reporter gene assay was used for transcription of CRYAA in the presence of a protective haplotype with individual SNP alteration, Chromatin immunoprecipitation (ChIP) was employed to determine whether SNPs regulated CRYAA expression by altering the binding affinity of transcription factors.

Results

Three polymorphisms were identified in the 5′ UTR of CRYAA: rs3761381 (P = 0.000357, odds ratio [OR] = 1.837), rs13053109 (P = 0.788, OR = 1.086), and rs7278468 (P = 0.00136, OR = 0.652). The haplotype C-G-T (P = 0.0014, OR = 1.536) increased the risk of nuclear ARC, whereas the haplotype T-G-G (P = 0.00029, OR = 0.535) decreased the risk. The haplotype C-G-T decreased CRYAA transcription through rs7278468, which is located in the binding site of specificity protein 1 (Sp1). Furthermore, the G allele of rs7278468 increased CRYAA transcription by enhancing the binding affinity of Sp1.

Conclusions

These data indicate that the CRYAA polymorphism is a genetic marker of inter-individual differences in the risk of nuclear ARC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12886-017-0529-9) contains supplementary material, which is available to authorized users.

CpG site methylation in CRYAA promoter affect transcription factor Sp1 binding in human lens epithelial cells

BACKGROUND

Age-related cataract (ARC) is the leading cause of visual impairment worldwide, and α-crystallin (CRYAA) is the predominant structural protein involved in the maintenance of lens clarity and refractive properties. We previously demonstrated that CRYAA genes undergo epigenetic repression in the lens epithelia in ARC. We further analyze the underlying mechanism in the current study.

METHODS

The transcription factor binding sites of the CpG island of CRYAA promoter were predicted by TESS website. An electrophoretic mobility shift assay (EMSA) was used to analyze the impact of the methylation of CpG sites on transcription factors. Human lens epithelial B-3 (HLE B-3) Cells were treated with demethylation agent zebularine in the concentrations of 0 (PBS as control), 10 μM, 20 μM, 50 μM, 100 μM and 200 μM, respectively. After treatment in the above concentrations for 24 h, 48 h and 72 h, respectively, CRYAA mRNA expression levels were detected by Quantitative Real-Time RT-PCR.

RESULTS

The methylation of the CpG site of the CRYAA promoter decreased the DNA-binding capacity of transcription factor Sp1. Zebularine increased CRYAA expression in HLE B-3 Cells in a dose- dependent and time- dependent pattern.

CONCLUSIONS

The evidence presented suggests that the methylation of the CpG sites of the CRYAA promotor directly affect Sp1 binding, leading to down expression of CRYAA in human lens epithelial cells. Zebularine treatment could restore CRYAA expression in a dose- dependent and time- dependent pattern.

Effect of SkQ1 eye drops on the rat lens metabolomic composition and the chaperone activity of α-crystallin

The ability of SkQ1 eye drops to slow down the cataract development is demonstrated on the senescence-accelerated OXYS rats: the SkQ1 treatment leads to the considerable improvement of the lens condition as compared to the control group. The comparison of the chaperone activities of α-crystallins isolated from the rat lenses did not reveal significant difference between SkQ1-treated and control rats. The contents of major metabolites (23 compounds) in lenses of SkQ1-treated and untreated rats are also very similar, though the concentration of reduced glutathione (GSH) in lenses of SkQ1-treated rats is 12% lower. This difference may be attributed to the reduction of the oxidative stress under action of SkQ1 eye drops, and to the decreased requirement to produce high amounts of this antioxidant.

An increase in phosphorylation and truncation of crystallin with the progression of cataracts

Background

Cataracts are the leading cause of blindness worldwide; however, there is no evidence regarding the direct formation of cataracts. At present, there is no treatment method other than surgery to prevent the formation or progression of cataracts.

Objective

Understanding the protein changes during various stages of cataracts might help realize the mechanism of the formation and progression of cataracts.

Methods

Lens materials were collected from cataract surgery. Cataracts were classified according to lens opacity using the gradation of the Lens Opacities Classification System. Lens proteins were separated by 2-dimensional polyacrylamide gel electrophoresis. Protein spots were visualized by Coomassie blue staining, and expression patterns were analyzed. Protein spots of interest were excised from 2-dimensional polyacrylamide gel electrophoresis gels, digested in situ with trypsin, and analyzed by mass spectrometry and liquid chromatographic tandem mass spectrometry.

Results

Crystallin was the major protein in the cataract lens, and αA, βB1, αB, and βA4 were the dominant types. Crystallin αB and βA4 increased with the formation of lens opacity. Moreover, phosphorylation and truncation of these proteins increased with the progression of cataracts.

Conclusion

Crystallin αB and βA4 and phosphorylation and truncation of crystallin in the lens might contribute to the formation of cataracts. In contrast, acetylation was not dominant in the progression of cataracts and did not play major role in the formation of cataracts.

Down-regulation and CpG island hypermethylation of CRYAA in age-related nuclear cataract

Age-related cataract (ARC) is the leading cause of blindness among older adults, and the chaperone-like activity of αA-crystallin (CRYAA) is considered to be critical for the maintenance of eye lens transparency. To assess the potential contribution of epigenetic regulation of CRYAA genes relevant to ARC pathogenesis, we evaluated DNA methylation, a tissue-specific genetic modulation that affects gene expression. Reverse-transcription PCR and Western blot were used to analyze the expression of CRYAA. Methylation status was analyzed by bisulfite genomic sequencing of the CpG islands in 15 eyes with age-related nuclear cataracts and 15 control eyes. The demethylating agent zebularine was used to investigate the relationship between hypermethylation of the CpG islands and down-expression of CRYAA. The mRNA and protein levels of CRYAA were significantly reduced in the lens epithelia of age-related nuclear cataract cases vs. age-matched controls, which corresponded to hypermethylation of the CpG island of CRYAA promoter. Treatment with a DNA-demethylating agent was associated with restoration of CRYAA expression. Comparing DNA methylation and mRNA and protein levels revealed significant differences between age-related nuclear cataract and control lenses. The evidence presented suggests that CRYAA undergoes epigenetic repression in the lens epithelia in age-related nuclear cataract.