Physiological Mechanisms Regulating Lens Transport

The role of the Hippo/YAP pathway in the physiological activities and lesions of lens epithelial cells

The Hippo/YAP pathway is a signaling pathway that plays an important role in cell proliferation, survival, differentiation, cell fate determination, organ size, and tissue homeostasis. Lens epithelial cells (LECs), located on the anterior surface of the lens, are the parental cells responsible for growth and development of the transparent ocular lens. During lens development, LECs undergo a process of differentiation where they exit the cell cycle and transform into lens fiber cells (LFCs), which constitute the majority of the lens structure. YAP is involved in the proliferation and differentiation of LECs, the maintenance of nuclear morphology, cell polarity, cell apical polarity complex, and connexin morphology. The role of the ordered arrangement of LFCs has been demonstrated in several animal studies, and Yap1 heterozygous deletion mice exhibit cataracts. The mechanism of the Hippo/YAP pathway in the physiological activities and lesions of LECs is complex, which is of great significance to understanding the development of the lens and the pathogenesis of lens-related diseases.

Combinatorial genetic manipulation of Cx50, PI3K and PTEN alters postnatal mouse lens growth and homeostasis

Introduction

Phosphoinositide 3-kinase (PI3K), Phosphatase and tensin homolog (PTEN) and connexin50 (Cx50) have individually been shown to play critical roles in the growth, development and maintenance of the lens and to functionally interact in vitro. To elucidate how gap junctional coupling mediated by Cx50 and intracellular signaling mediated by PI3K and PTEN synergistically interact to regulate lens homeostasis in vivo, we generated and characterized double knockout animal models lacking the p110α subunit of PI3K and Cx50, or PTEN and Cx50.

Methods

We interbred lens specific p110α and PTEN conditional knockout animals with Cx50 deficient mice to generate double knockouts. Animals and eyes were weighed, lenses were dissected, photographed, measured, fixed and sectioned for histological analysis. Lens epithelial cell proliferation was determined using 5-ethynyl-2'-deoxyuridine (EdU) labeling.

Results

Double knockout of p110α and Cx50 led to a significant reduction in lens and eye size, and a high rate of lens rupture. The individual cell proliferation defects of the Cx50 and p110α single knockout lenses both persisted in the double KO. Double deletion of Cx50 and PTEN produced severe lens defects, including cataract, aberrant cell migration, altered cell proliferation, vacuole formation and lens rupture.

Conclusion

The severe phenotypes in p110α/Cx50 and PTEN/Cx50 double deficient lenses suggest that PI3K, PTEN and Cx50 participate in both distinct and common regulatory pathways that are necessary to maintain normal lens growth and homeostasis.

Ocular inoculation of toad venom: toxic cataract and proteomic profiling

Purpose

To report a singular case of cataract caused by toad venom inoculation and to scrutinize the pathological mechanisms through proteomic sequencing of the lens specimen.

Methods

A young Chinese male presented with progressively deteriorating vision in his right eye subsequent to a history of toad venom inoculation. He was diagnosed with a toxic cataract, and underwent phacoemulsification cataract surgery. Anterior capsule, nucleus, and cortex specimens from the patient (designated as PT_CAP, PT_PHACO, and PT_CTX, respectively) and age-related cataract controls (C_CAP, C_PHACO, and C_CTX, respectively) were collected and subjected to 4D label-free quantitative proteomics.

Results

A multitude of differentially expressed proteins (DEPs) were identified in the patient's lens compared to those in the controls. Specifically, a total of 204 DEPs were identified in PT_CAP compared to C_CAP, with MYH6, MYL2, MYL3, STAT1, and ANK1 among the foremost regulated DEPs. The DEPs of PT_CAP were principally affiliated with functions including "transportation of small molecules," "regulation of metal ion transport," and "import into cell." A sum of 109 DEPs were delineated in PT_CTX compared to C_CTX, with TPM1 among the top-10 downregulated DEPs. Ninety-five DEPs were pinpointed in PT_PHACO compared to C_PHACO, with hexokinase among the top 10 downregulated DEPs. These proteins were ascertained to be linked with Na+/K+-ATPase activity.

Conclusion

This study introduced the first documented case of toxic cataract caused by toad venom inoculation. Proteomic sequencing indicated a correlation between cataract and alterations in Na+/K+-ATPase activity, providing insights for the clinical management of ocular toad venom inoculation in subsequent cases.

Influence of zonular tension on molecular transport in the porcine ocular lens

Introduction

Accommodation is the process of changing the ocular lens' refractive power and focal distance. This process involves application of biomechanical forces on the lens by the surrounding musculature. Previous studies have demonstrated that the lens epithelium demonstrates mechanotransduction and that tension influences its chemical activity. It is not yet known how these forces affect the structure and permeability of the lens. This study aimed to identify the influence of tension on molecular transport of dyes through the lens.

Methods

Paired porcine eyes were incubated in each of four dyes for three time periods with no stretch (null), static, or cyclic stretching using a bespoke mechanical lens stretcher. After incubation, the lenses were frozen and cryosectioned sagittally through the optic axis. Photographs of the stretched and unstretched lenses were compared and qualitatively assessed.

Results

None of the four dyes showed drastic stretch-induced differences in dye penetration depth. However, the dye neutral red showed dramatic stretch-induced changes in the dye uptake color behind lens anterior surfaces, with unstretched lenses appearing far more orange than their stretched counterparts. Three of four dyes showed notable differences between anterior and posterior diffusion patterns. One dye, methylene blue, demonstrated unexpected intensity in the lens nucleus compared to the lower intensity shown in the cortex, suggesting active transport rather than a linearly graded passive diffusion regardless of stretching condition.

Discussion

All this taken together suggests that lens transport is more complex than simple passive diffusion and that active transport of some molecules may be affected by stretching. Future work should assess the mechanisms of transport for the various dyes and attempt to explain the dye permeation patterns observed here, including the effects of stretching.

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.

The significance of growth shells in development of symmetry, transparency, and refraction of the human lens

Human visual function depends on the biological lens, a biconvex optical element formed by coordinated, synchronous generation of growth shells produced from ordered cells at the lens equator, the distal edge of the epithelium. Growth shells are comprised of straight (St) and S-shaped (SSh) lens fibers organized in highly symmetric, sinusoidal pattern which optimizes both the refractile, transparent structure and the unique microcirculation that regulates hydration and nutrition over the lifetime of an individual. The fiber cells are characterized by diversity in composition and age. All fiber cells remain interconnected in their growth shells throughout the life of the adult lens. As an optical element, cellular differentiation is constrained by the physical properties of light and its special development accounts for its characteristic symmetry, gradient of refractive index (GRIN), short range transparent order (SRO), and functional longevity. The complex sinusoidal structure is the basis for the lens microcirculation required for the establishment and maintenance of image formation.

Through the Cat-Map Gateway: A Brief History of Cataract Genetics

Clouding of the transparent eye lens, or cataract(s), is a leading cause of visual impairment that requires surgical replacement with a synthetic intraocular lens to effectively restore clear vision. Most frequently, cataract is acquired with aging as a multifactorial or complex trait. Cataract may also be inherited as a classic Mendelian trait-often with an early or pediatric onset-with or without other ocular and/or systemic features. Since the early 1990s, over 85 genes and loci have been genetically associated with inherited and/or age-related forms of cataract. While many of these underlying genes-including those for lens crystallins, connexins, and transcription factors-recapitulate signature features of lens development and differentiation, an increasing cohort of unpredicted genes, including those involved in cell-signaling, membrane remodeling, and autophagy, has emerged-providing new insights regarding lens homeostasis and aging. This review provides a brief history of gene discovery for inherited and age-related forms of cataract compiled in the Cat-Map database and highlights potential gene-based therapeutic approaches to delay, reverse, or even prevent cataract formation that may help to reduce the increasing demand for cataract surgery.

Aquaporins contribute to vacuoles formation in Nile grass type II diabetic rats

Regulation of ion and water microcirculation within the lens is tightly controlled through aquaporin channels and connexin junctions. However, cataracts can occur when the lens becomes cloudy. Various factors can induce cataracts, including diabetes which is a well-known cause. The most common phenotype of diabetic cataracts is a cortical and/or posterior subcapsular opacity. In addition to the three main types and two subtypes of cataracts, a vacuole formation is frequently observed; however, their origin remains unclear. In this study, we focused on the aquaporins and connexins involved in diabetes-induced cataracts and vacuoles in Nile grass type II diabetes. The results showed that the expression of aquaporin 0 and aquaporin 5 increased, and that of connexin 43 decreased in diabetic rat lenses. Additionally, aquaporin 0 and 5 were strongly localized in peripheral of vacuoles, suggesting that aquaporins are involved in vacuoles formation. Transillumination photography revealed large vacuoles at the tip of the Y-suture in the anterior capsule of the diabetic lens, and several small vacuoles were observed in the posterior capsule. Within the vacuoles, cytoplasmic degradation and aggregation of fibrous material were observed. Our findings suggest that aquaporins are potential candidate proteins for preventing vacuole formation.

Single-Cell RNA Sequencing Analysis of the Early Postnatal Mouse Lens Epithelium

Purpose

The lens epithelium maintains the overall health of the organ. We used single-cell RNA sequencing (scRNA-seq) technology to assess transcriptional heterogeneity between cells in the postnatal day 2 (P2) epithelium and identify distinct epithelial cell subtypes. Analysis of these data was used to better understand lens growth, differentiation, and homeostasis on P2.

Methods

scRNA-seq on P2 mouse lenses was performed using the 10x Genomics Chromium Single Cell 3' Kit (v3.1) and short-read Illumina sequencing. Sequence alignment and preprocessing of data were conducted using 10x Genomics Cell Ranger software. Seurat was employed for preprocessing, quality control, dimensionality reduction, and cell clustering, and Monocle was utilized for trajectory analysis to understand the developmental progression of the lens cells. CellChat and GO analyses were used to explore cell-cell communication networks and signaling interactions.

Results

Lens epithelial cells (LECs) were divided into seven subclusters, classified by specific gene markers. The expression of crystallin, cell-cycle, and metabolic genes was not uniform, indicating distinct functional roles of LECs. Trajectory analysis predicted a bifurcation of differentiating and cycling cells from an Igfbp5+ progenitor pool. We also identified heterogeneity in signaling molecules and pathways, suggesting that cycling and progenitor subclusters have prominent roles in coordinating crosstalk.

Conclusions

scRNA-seq corroborated many known markers of epithelial differentiation and proliferation while providing further insight into the pathways and genes directing these processes. Interestingly, we demonstrated that the developing epithelium can be divided into distinct subpopulations. These clusters reflect the transcriptionally diverse roles of the epithelium in proliferation, signaling, and maintenance.

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.

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.

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Double Deletion of PI3K and PTEN Modifies Lens Postnatal Growth and Homeostasis

We have previously shown that the conditional deletion of either the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K), or its opposing phosphatase, phosphatase and tensin homolog (PTEN), had distinct effects on lens growth and homeostasis. The deletion of p110α reduced the levels of phosphorylated Akt and equatorial epithelial cell proliferation, and resulted in smaller transparent lenses in adult mice. The deletion of PTEN increased levels of phosphorylated Akt, altered lens sodium transport, and caused lens rupture and cataract. Here, we have generated conditional p110α/PTEN double-knockout mice, and evaluated epithelial cell proliferation and lens homeostasis. The double deletion of p110α and PTEN rescued the defect in lens size seen after the single knockout of p110α, but accelerated the lens rupture phenotype seen in PTEN single-knockout mice. Levels of phosphorylated Akt in double-knockout lenses were significantly higher than in wild-type lenses, but not as elevated as those reported for PTEN single-knockout lenses. These results showed that the double deletion of the p110α catalytic subunit of PI3K and its opposing phosphatase, PTEN, exacerbated the rupture defect seen in the single PTEN knockout and alleviated the growth defect observed in the single p110α knockout. Thus, the integrity of the PI3K signaling pathway was absolutely essential for proper lens homeostasis, but not for lens growth.

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.