Connexin Gap Junctions and Hemichannels in Modulating Lens Redox Homeostasis and Oxidative Stress in Cataractogenesis

Crosstalk between signaling pathways (Rho/ROCK, TGF-β and Wnt/β-Catenin Pathways/ PI3K-AKT-mTOR) in Cataract: A Mechanistic Exploration and therapeutic strategy

Cataract are a leading cause of visual impairment that is characterized by clouding or lens opacification of the healthy clear lens of the eye or its capsule. It can be classified based on their etiology and clinical presentation such as congenital, age-related, and secondary cataracts. Clinically, it may be further classified as a cortical or nuclear cataract. Cortical cataracts are responsible for opacification of the lens cortex, while nuclear cataracts cause age-related degeneration of the lens nucleus. This review aims to explore the molecular mechanism associated with various signaling pathways underlying cataract formation. Additionally, explore the potential therapeutic strategies for the management of cataracts. A comprehensive literature search was performed utilizing different keywords such as cataract, pathogenesis, signaling pathways, therapeutic approaches, RNA therapeutics, and surgery. Electronic databases such as PubMed, Google Scholar, Springer Link, and Web of Science were used for the literature search. The cataract formation is responsible for protein aggregation, primarily of γ-crystallin, and causes disruptions in signaling pathways. Key pathways include Rho/ROCK, TGF-β, Wnt/β-catenin, NF-κB, and PI3K-AKT-mTOR. Signaling pathways governing lens epithelial cell differentiation and epithelial-to-mesenchymal transition (EMT) are essential for maintaining lens transparency. Disruptions in these pathways, often caused by genetic mutations in genes like MIP, TDRD7, PAX6, FOXE3, HSF4, MAF, and PITX3 lead to cataract formation. While surgical intervention remains the primary treatment, pharmacological therapies and emerging RNA-based strategies offer promising strategies for the prevention and management of cataracts. A deeper understanding of the underlying molecular mechanisms is essential to develop innovative therapeutic strategies and improve the quality of life for individuals affected by cataracts.

Ambient air pollution exposure and incidence of cataract surgery: The prospective 3City-Alienor study

PURPOSE

Cataract, the leading cause of blindness worldwide, is a multifactorial disease involving oxidative stress mechanisms. The aim of our study was to investigate the relationship between air pollution exposure and the incidence of cataract surgery.

METHODS

The 3C-Alienor study is a population-based cohort of residents of Bordeaux, France, aged 65 years or more, recruited in 1999-2000 and followed every 2-3 years until 2017. Cataract surgery was self-reported and checked at slit-lamp by trained professionals. Average air pollution exposure (particulate matter ≤2.5 μm (PM2.5), black carbon (BC), nitrogen dioxide (NO2)) in the 10 years preceding baseline was estimated at the participants' geocoded residential address, using temporally adjusted land use regression. Associations of 10-year average air pollution exposure with incidence of cataract were estimated using Cox proportional hazard models adjusted for confounders.

RESULTS

The study included 829 subjects without cataract surgery prior to inclusion; the mean age at inclusion was 72.6 years (standard deviation (SD): 4.2) and 61% were women. The median (Interquartile-range (IQR)) follow-up duration was 14.1 (6.4) years during which 507 participants underwent cataract surgery. Exposure to a concentration ≥40 μg/m3 of NO2 (the current regulatory limit value in Europe) was associated with incident cataract surgery (HR = 1.46, CI (1.16, 1.84), p = 0.001). No statistically significant association was found with PM2.5 and BC.

CONCLUSION

Long-term exposure to a NO2 concentration ≥ 40 μg/m3 was associated with an increased incidence of cataract surgery. Complying with current European air pollution standards could reduce cataract surgery costs and improve population quality of life.

A novel GJA3 mutation causing autosomal dominant congenital perinuclear cataracts

OBJECTIVE

To identify the cause of congenital perinuclear cataracts in a Chinese family and its underlying mechanism.

METHODS

Family history and clinical data were recorded, and candidate genes were amplified by polymerase chain reaction (PCR) and screened for mutations using direct bidirectional DNA sequencing. The GJA3 gene was acquired from a human lens cDNA library, and the GJA3 mutant was generated by PCR-based site-directed mutagenesis. Connexin localization and gap junction formation were assessed by fluorescence microscopy, and hemichannel functions were analyzed by dye uptake assay.

RESULTS

Gene sequencing showed one base pair substitution at position 671 of the GJA3 gene's coding region (c.671A > G), leading to the conversion of the 224th amino acid of the Connexin 46 protein (Cx46), expressed by the GJA3 gene, from histidine to arginine (p.H224R). In stable transfectants, the formation of gap junctions was detected in both wild-type Cx46 (wtCx46) and mutant Cx46H224R transfected HeLa cells, where the Cx46H224R transfected cells exhibited a much higher Propidium Iodide (PI) loading speed than the wtCx46 cells.

CONCLUSION

This study was the first to identify the c. 671A > G mutation of the GJA3 gene (p.H224R in Cx46), which leads to the generation of congenital perinuclear cataracts. We suggest that the H224R missense mutation of Cx46 may cause alterations in the activity of the hemichannel, leading to cataract development.

Gold nanoparticles and induction of structural alteration and enhanced oxidative stress in rat lens

There is an emerging wide use of nanotechnology in the medical fields. The information regarding distribution and clearance of gold nanoparticles (AuNPs) in the ocular tissue is insufficient. We investigated the cumulative effect of AuNPs on rat lens structure and their effect on the redox state and aquaporin-0 (AQP0) expression. Thirty-six male rats were distributed as follow: control, AuNPs-200 (200 μg/kg/rat for 4-weeks) and AuNPs-500 (500 μg/kg/rat for 4-weeks) groups. Rats were euthanized after 4-weeks, and the eye lenses were investigated for histological studies, transmission and scanning electron microscopic studies, immunohistochemistry for AQP0 and morphometric measures. Lens homogenates were investigated for tumour necrosis factor-alpha (TNF-α) and total reactive oxygen species levels by ELISA and for p-c-SRC by western-blot. AuNPs administration induced morphological and ultrastructural changes in rat lens. Degenerative changes in the lens epithelium, cytoplasmic vacuoles, distorted separated cortical lens fibers and loss of ball-and-socket junctions were observed. A significant reduction of AQP0-immune-staining with a significant elevation of TNF-α, total ROS and p-c-SRC content in rat lens homogenates were detected as compared to the control group. Repetitive spherical 20 nm-sized AuNPs administration, especially at 500 μg/kg/rat, induced structural changes in lens fibers of rats and increased oxidative stress level in the lens tissue.

Peroxiredoxin 6 maintains mitochondrial homeostasis and promotes tumor progression through ROS/JNK/p38 MAPK signaling pathway in multiple myeloma

Peroxiredoxin 6 (PRDX6) is one of the Peroxiredoxin family members with only 1-Cys, using glutathione as the electron donor to reduce peroxides in cells. PRDX6 has been frequently studied and its expression was associated with poor prognosis in many tumors. However, the expression of PRDX6 in multiple myeloma (MM) and its relevance with MM remain unclear. In our study, we found that PRDX6 was overexpressed in MM patients. Its high expression was inversely correlated with prognosis but positively correlated with the levels of β2-microglobulin (B2M), lactate dehydrogenase (LDH), and International Staging System (ISS) stage of MM patients. Further, the deficiency of PRDX6 promoted MM cell lines (RPMI 8226, MM.1S, and U266) apoptosis significantly. Mechanically, PRDX6 serves as an anti-oxidative enzyme, and its deficiency led to over-accumulation of reactive oxygen species (ROS), resulting in oxidative stress, following the activation of MAPK signaling pathway, which manifested as phosphorylation of JNK and p38. Then, the expression of BAX and Bcl2 was imbalance, and the cascade cleavage of PARP and caspase 3 was increased, ultimately triggering cell apoptosis. In addition, oxidative stress decreased mitochondrial membrane potential (MMP), reduced gene expression levels of oxidative phosphorylation (OXPHOS), and increased in the density of mitochondrial crumpling, leading to mitochondrial structural abnormalities and dysfunction. Furthermore, PRDX6 deficiency combined with bortezomib induced a robust anti-tumor effect in MM cell lines. Finally, in vivo experiments also showed that the deficiency of PRDX6 inhibited tumor growth of tumor-bearing mice. Collectively, PRDX6 protects MM cells from oxidative damage and maintains mitochondrial homeostasis. And targeting PRDX6 is an attractive strategy to enhance the anti-tumor effect of bortezomib in MM.

Effect of hydrogen peroxide on lens transparency, intracellular pH, gap junction coupling, hydrostatic pressure and membrane water permeability

Clouding of the eye lens or cataract is an age-related anomaly that affects middle-aged humans. Exploration of the etiology points to a great extent to oxidative stress due to different forms of reactive oxygen species/metabolites such as Hydrogen peroxide (H2O2) that are generated due to intracellular metabolism and environmental factors like radiation. If accumulated and left unchecked, the imbalance between the production and degradation of H2O2 in the lens could lead to cataracts. Our objective was to explore ex vivo the effects of H2O2 on lens physiology. We investigated transparency, intracellular pH (pHi), intercellular gap junction coupling (GJC), hydrostatic pressure (HP) and membrane water permeability after subjecting two-month-old C57 wild-type (WT) mouse lenses for 3 h or 8 h in lens saline containing 50 μM H2O2; the results were compared with control lenses incubated in the saline without H2O2. There was a significant decrease in lens transparency in H2O2-treated lenses. In control lenses, pHi decreases from ∼7.34 in the surface fiber cells to 6.64 in the center. Experimental lenses exposed to H2O2 for 8 h showed a significant decrease in surface pH (from 7.34 to 6.86) and central pH (from 6.64 to 6.56), compared to the controls. There was a significant increase in GJC resistance in the differentiating (12-fold) and mature (1.4-fold) fiber cells compared to the control. Experimental lenses also showed a significant increase in HP which was ∼2-fold higher at the junction between the differentiating and mature fiber cells and ∼1.5-fold higher at the center compared to these locations in control lenses; HP at the surface was 0 mm Hg in either type lens. Fiber cell membrane water permeability significantly increased in H2O2-exposed lenses compared to controls. Our data demonstrate that elevated levels of lens intracellular H2O2 caused a decrease in intracellular pH and led to acidosis which most likely uncoupled GJs, and increased AQP0-dependent membrane water permeability causing a consequent rise in HP. We infer that an abnormal increase in intracellular H2O2 could induce acidosis, cause oxidative stress, alter lens microcirculation, and lead to the development of accelerated lens opacity and age-related cataracts.

Mechano-activated connexin hemichannels and glutathione transport protect lens fiber cells against oxidative insults

Long-lived lens fiber cells require a robust cellular protective function against oxidative insults to maintain their hemostasis and viability; however, the underlying mechanism is largely obscure. In this study, we unveiled a new mechanism that protects lens fiber cells against oxidative stress-induced cell death. We found that mechano-activated connexin (Cx) hemichannels (HCs) mediate the transport of glutathione (GSH) into chick embryonic fibroblasts (CEF) and primary lens fiber cells, resulting in a decrease in the accumulation of intracellular reactive oxygen species induced by both H2O2 and ultraviolet B, providing protection to lens fiber cells against cell apoptosis and necrosis. Furthermore, HCs formed by both homomeric Cx50 or Cx46 and heteromeric Cx50/Cx46 were mechanosensitive and could transport GSH into CEF cells. Notably, mechano-activated Cx50 HCs exhibited a greater capacity to transport GSH than Cx46 HCs. Consistently, the deficiency of Cx50 in single lens fiber cells led to a higher level of oxidative stress. Additionally, outer cortical short lens fiber cells expressing full length Cxs demonstrated greater resistance to oxidative injury compared to central core long lens fibers. Taken together, our results suggest that the activation of Cx HCs by interstitial fluid flow in cultured epithelial cells and isolated fiber cells shows that HCs can serve as a pathway for moving GSH across the cell membrane to offer protection against oxidative stress.

A bibliometric and visualized analysis of the pathogenesis of cataracts from 1999 to 2023

Research on the pathogenesis of cataracts is ongoing and the number of publications on this topic is increasing annually. This study offers an overview of the research status, popular topics, and scholarly tendencies in the field of cataract pathogenesis over recent decades，which helps to guide future research directions, and optimize resource allocation. In the present study, we performed a bibliometric analysis of cataract pathogenesis. Publications from January 1, 1999, to December 20, 2023, were collected from the Web of Science Core Collection (WoSCC), and the extracted data were quantified and analyzed. We analyzed and presented the data using Microsoft Excel, VOSviewer, CiteSpace, and Python. In all, 4006 articles were evaluated based on various characteristics, including publication year, authors, countries, institutions, journals, citations, and keywords. This study utilized VOSviewer to conduct visualized analysis, including co-authorship, co-citation, co-occurrence, and network visualization. The CiteSpace software was used to identify keywords with significant bursts of activity. The number of annual global publications climbed from 76 to 277 between 1999 and 2023, a 264.47% rise. Experimental Eye Research published the most manuscripts (178 publications), whereas Investigative Ophthalmology & Visual Science received the most citations (6675 citations). The most influential and productive country, institution, and author were the United States (1244 publications, 54,456 citations), University of California system (136 publications, 5401 citations), and Yao Ke (49 publications, 838 citations), respectively. The top 100 ranked keywords are divided into four clusters through co-occurrence analysis: (1) secondary cataracts, (2) oxidative stress, (3) gene mutations and protein abnormalities, and (4) alteration of biological processes in lens epithelial cells. Further discussions on the four subtopics outline the research topics and trends. In conclusion, the specific mechanism of cataract formation remains a popular topic for future research and should be explored in greater depth.

Antioxidant potential of chlorogenic acid in Age-Related eye diseases

Oxidative stress is an important mechanism of aging, and in turn, aging can also aggravate oxidative stress, which leads to a vicious cycle. In the process of the brain converting light into visual signals, the eye is stimulated by harmful blue-light radiation directly. Thus, the eye is especially vulnerable to oxidative stress and becomes one of the organs most seriously involved during the aging process. Cataracts, age-related macular degeneration (AMD), glaucoma, diabetic retinopathy (DR), and dry eye are inextricably linked to the aging process and oxidative stress. Chlorogenic acid (CGA) has been demonstrated to have antioxidant and anti-inflammatory activities, and its validity has been established experimentally in numerous fields, including cardiovascular disease, metabolic disorders, cancers, and other chronic diseases. There has previously been evidence of CGA's therapeutic effect in the field of ophthalmopathy. Considering that many ophthalmic drugs lead to systemic side effects, CGA may act as a natural exogenous antioxidant for patients to take regularly, controlling their condition while minimizing side effects. In this paper, in vitro and in vivo studies of CGA in the treatment of age-related eye diseases are reviewed, and the prospects of CGA's antioxidant application for the eye are discussed. The aim of this review is to summarize the relevant knowledge and provide theoretical support for future research.

JAM-C Is Important for Lens Epithelial Cell Proliferation and Lens Fiber Maturation in Murine Lens Development

Purpose

The underlying mechanism of congenital cataracts caused by deficiency or mutation of junctional adhesion molecule C (JAM-C) gene remains unclear. Our study aims to elucidate the abnormal developmental process in Jamc-/- lenses and reveal the genes related to lens development that JAM-C may regulate.

Methods

Jamc knockout (Jamc-/-) mouse embryos and pups were generated for in vivo studies. Four key developmental stages from embryonic day (E) 12.5 to postnatal day (P) 0.5 were selected for the following experiments. Hematoxylin and eosin staining was used for histological analysis. The 5-bromo-2'-deoxyuridine (BrdU) incorporation assay and TUNEL staining were performed to label lens epithelial cell (LEC) proliferation and apoptosis, respectively. Immunofluorescence and Western blot were used to analyze the markers of lens epithelium, cell cycle exit, and lens fiber differentiation.

Results

JAM-C was expressed throughout the process of lens development. Deletion of Jamc resulted in decreased lens size and disorganized lens fibers, which arose from E16.5 and aggravated gradually. The LECs of Jamc-/- lenses showed decreased quantity and proliferation, accompanied with reduction of key transcription factor, FOXE3. The fibers in Jamc-/- lenses were disorganized. Moreover, Jamc-deficient lens fibers showed significantly altered distribution patterns of Cx46 and Cx50. The marker of fiber homeostasis, γ-crystallin, was also decreased in the inner cortex and core fibers of Jamc-/- lenses.

Conclusions

Deletion of JAM-C exhibits malfunction of LEC proliferation and fiber maturation during murine lens development, which may be related to the downregulation of FOXE3 expression and abnormal localization patterns of Cx46 and Cx50.

The Role of Gap Junctions Dysfunction in the Development of Cataracts: From Loss of Cell-to-Cell Transfer to Blurred Vision-Review

Mutations of lens connexins are linked to congenital cataracts. However, the role of connexin mutations in the development of age-related lens opacification remains largely unknown. Here, we present a focused review of the literature on lens organization and factors associated with cataract development. Several lines of evidence indicate that disturbances of the lens circulation by dysfunctional connexin channels, and/or accumulation of protein damage due to oxidative stress, are key factors in cataract development. Phosphorylation by protein kinase A improves the permeability of connexins channels to small molecules and mitigates the lens clouding induced by oxidative stress. We conclude (1) that connexin channels are central to the lens circulation and (2) that their permeability to antioxidant molecules contributes to the maintenance of lens transparency.

Whole Exome Sequencing of 20 Spanish Families: Candidate Genes for Non-Syndromic Pediatric Cataracts

Non-syndromic pediatric cataracts are defined as opacification of the crystalline lens that occurs during the first years of life without affecting other organs. Given that this disease is one of the most frequent causes of reversible blindness in childhood, the main objective of this study was to propose new responsible gene candidates that would allow a more targeted genetic approach and expand our genetic knowledge about the disease. We present a whole exome sequencing (WES) study of 20 Spanish families with non-syndromic pediatric cataracts and a previous negative result on an ophthalmology next-generation sequencing panel. After ophthalmological evaluation and collection of peripheral blood samples from these families, WES was performed. We were able to reach a genetic diagnosis in 10% of the families analyzed and found genes that could cause pediatric cataracts in 35% of the cohort. Of the variants found, 18.2% were classified as pathogenic, 9% as likely pathogenic, and 72.8% as variants of uncertain significance. However, we did not find conclusive results in 55% of the families studied, which suggests further studies are needed. The results of this WES study allow us to propose LONP1, ACACA, TRPM1, CLIC5, HSPE1, ODF1, PIKFYVE, and CHMP4A as potential candidates to further investigate for their role in pediatric cataracts, and AQP5 and locus 2q37 as causal genes.

Autophagy Requirements for Eye Lens Differentiation and Transparency

Recent evidence points to autophagy as an essential cellular requirement for achieving the mature structure, homeostasis, and transparency of the lens. Collective evidence from multiple laboratories using chick, mouse, primate, and human model systems provides evidence that classic autophagy structures, ranging from double-membrane autophagosomes to single-membrane autolysosomes, are found throughout the lens in both undifferentiated lens epithelial cells and maturing lens fiber cells. Recently, key autophagy signaling pathways have been identified to initiate critical steps in the lens differentiation program, including the elimination of organelles to form the core lens organelle-free zone. Other recent studies using ex vivo lens culture demonstrate that the low oxygen environment of the lens drives HIF1a-induced autophagy via upregulation of essential mitophagy components to direct the specific elimination of the mitochondria, endoplasmic reticulum, and Golgi apparatus during lens fiber cell differentiation. Pioneering studies on the structural requirements for the elimination of nuclei during lens differentiation reveal the presence of an entirely novel structure associated with degrading lens nuclei termed the nuclear excisosome. Considerable evidence also indicates that autophagy is a requirement for lens homeostasis, differentiation, and transparency, since the mutation of key autophagy proteins results in human cataract formation.

The lens epithelium as a major determinant in the development, maintenance, and regeneration of the crystalline lens

The crystalline lens is a transparent and refractive biconvex structure formed by lens epithelial cells (LECs) and lens fibers. Lens opacity, also known as cataracts, is the leading cause of blindness in the world. LECs are the principal cells of lens throughout human life, exhibiting different physiological properties and functions. During the embryonic stage, LECs proliferate and differentiate into lens fibers, which form the crystalline lens. Genetics and environment are vital factors that influence normal lens development. During maturation, LECs help maintain lens homeostasis through material transport, synthesis and metabolism as well as mitosis and proliferation. If disturbed, this will result in loss of lens transparency. After cataract surgery, the repair potential of LECs is activated and the structure and transparency of the regenerative tissue depends on postoperative microenvironment. This review summarizes recent research advances on the role of LECs in lens development, homeostasis, and regeneration, with a particular focus on the role of cholesterol synthesis (eg., lanosterol synthase) in lens development and homeostasis maintenance, and how the regenerative potential of LECs can be harnessed to develop surgical strategies and improve the outcomes of cataract surgery (Fig. 1). These new insights suggest that LECs are a major determinant of the physiological and pathological state of the lens. Further studies on their molecular biology will offer possibility to explore new approaches for cataract prevention and treatment.

Role and Posttranslational Regulation of Cx46 Hemichannels and Gap Junction Channels in the Eye Lens

Connexins are a family of proteins that can form two distinct types of channels: hemichannels and gap junction channels. Hemichannels are composed of six connexin subunits and when open allow for exchanges between the cytoplasm and the extracellular milieu. Gap junction channels are formed by head-to-head docking of two hemichannels in series, each one from one of two adjacent cells. These channels allow for exchanges between the cytoplasms of contacting cells. The lens is a transparent structure located in the eye that focuses light on the retina. The transparency of the lens depends on its lack of blood irrigation and the absence of organelles in its cells. To survive such complex metabolic scenario, lens cells express Cx43, Cx46 and Cx50, three connexins isoforms that form hemichannels and gap junction channels that allow for metabolic cooperation between lens cells. This review focuses on the roles of Cx46 hemichannels and gap junction channels in the lens under physiological conditions and in the formation of cataracts, with emphasis on the modulation by posttranslational modifications.

Physiological Mechanisms Regulating Lens Transport

The transparency and refractive properties of the lens are maintained by the cellular physiology provided by an internal microcirculation system that utilizes spatial differences in ion channels, transporters and gap junctions to establish standing electrochemical and hydrostatic pressure gradients that drive the transport of ions, water and nutrients through this avascular tissue. Aging has negative effects on lens transport, degrading ion and water homeostasis, and producing changes in lens water content. This alters the properties of the lens, causing changes in optical quality and accommodative amplitude that initially result in presbyopia in middle age and ultimately manifest as cataract in the elderly. Recent advances have highlighted that the lens hydrostatic pressure gradient responds to tension transmitted to the lens through the Zonules of Zinn through a mechanism utilizing mechanosensitive channels, multiple sodium transporters respond to changes in hydrostatic pressure to restore equilibrium, and that connexin hemichannels and diverse intracellular signaling cascades play a critical role in these responses. The mechanistic insight gained from these studies has advanced our understanding of lens transport and how it responds and adapts to different inputs both from within the lens, and from surrounding ocular structures.